Add HashiCorp Vault key manager plugin to SPIRE server

doc/plugin_server_keymanager_hashicorp_vault.md

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+# Server plugin: KeyManager "hashicorp_vault"
+
+The `hashicorp_vault` key manager plugin leverages HashiCorp Vault to create, maintain, and rotate key pairs, signing
+SVIDs as needed.
+
+## Configuration
+
+The plugin accepts the following configuration options:
+
+| key                  | type   | required | description                                                                                              | default              |
+|:---------------------|:-------|:---------|:---------------------------------------------------------------------------------------------------------|:---------------------|
+| vault_addr           | string |          | The URL of the Vault server. (e.g., <https://vault.example.com:8443/>)                                   | `${VAULT_ADDR}`      |
+| namespace            | string |          | Name of the Vault namespace. This is only available in the Vault Enterprise.                             | `${VAULT_NAMESPACE}` |
+| transit_engine_path  | string |          | Path of the transit engine that stores the keys.                                                         | transit              |
+| ca_cert_path         | string |          | Path to a CA certificate file used to verify the Vault server certificate. Only PEM format is supported. | `${VAULT_CACERT}`    |
+| insecure_skip_verify | bool   |          | If true, vault client accepts any server certificates                                                    | false                |
+| cert_auth            | struct |          | Configuration for the Client Certificate authentication method                                           |                      |
+| token_auth           | struct |          | Configuration for the Token authentication method                                                        |                      |
+| approle_auth         | struct |          | Configuration for the AppRole authentication method                                                      |                      |
+| k8s_auth             | struct |          | Configuration for the Kubernetes authentication method                                                   |                      |
+
+The plugin supports **Client Certificate**, **Token** and **AppRole** authentication methods.
+
+- **Client Certificate** method authenticates to Vault using a TLS client certificate.
+- **Token** method authenticates to Vault using the token in a HTTP Request header.
+- **AppRole** method authenticates to Vault using a RoleID and SecretID that are issued from Vault.
+
+The [`ca_ttl` SPIRE Server configurable](https://github.com/spiffe/spire/blob/main/doc/spire_server.md#server-configuration-file)
+should be less than or equal to the Vault's PKI secret engine TTL.
+To configure the TTL value, tune the engine.
+
+e.g.
+
+```shell
+$ vault secrets tune -max-lease-ttl=8760h pki
+```
+
+The configured token needs to be attached to a policy that has at least the following capabilities:
+
+```hcl
+path "pki/root/sign-intermediate" {
+  capabilities = ["update"]
+}
+```
+
+## Client Certificate Authentication
+
+| key                   | type   | required | description                                                                                                          | default                |
+|:----------------------|:-------|:---------|:---------------------------------------------------------------------------------------------------------------------|:-----------------------|
+| cert_auth_mount_point | string |          | Name of the mount point where TLS certificate auth method is mounted                                                 | cert                   |
+| cert_auth_role_name   | string |          | Name of the Vault role. If given, the plugin authenticates against only the named role. Default to trying all roles. |                        |
+| client_cert_path      | string |          | Path to a client certificate file. Only PEM format is supported.                                                     | `${VAULT_CLIENT_CERT}` |
+| client_key_path       | string |          | Path to a client private key file. Only PEM format is supported.                                                     | `${VAULT_CLIENT_KEY}`  |
+
+```hcl
+    KeyManager "hashicorp_vault" {
+        plugin_data {
+            vault_addr = "https://vault.example.org/"
+            pki_mount_point = "test-pki"
+            ca_cert_path = "/path/to/ca-cert.pem"
+            cert_auth {
+                cert_auth_mount_point = "test-tls-cert-auth"
+                client_cert_path = "/path/to/client-cert.pem"
+                client_key_path  = "/path/to/client-key.pem"
+            }
+            // If specify the role to authenticate with
+            // cert_auth {
+            //     cert_auth_mount_point = "test-tls-cert-auth"
+            //     cert_auth_role_name = "test"
+            //     client_cert_path = "/path/to/client-cert.pem"
+            //     client_key_path  = "/path/to/client-key.pem"
+            // }
+
+            // If specify the key-pair as an environment variable and use the modified mount point
+            // cert_auth {
+            //    cert_auth_mount_point = "test-tls-cert-auth"
+            // }
+
+            // If specify the key-pair as an environment variable and use the default mount point, set the empty structure.
+            // cert_auth {}
+        }
+    }
+```
+
+## Token Authentication
+
+| key   | type   | required | description                                     | default          |
+|:------|:-------|:---------|:------------------------------------------------|:-----------------|
+| token | string |          | Token string to set into "X-Vault-Token" header | `${VAULT_TOKEN}` |
+
+```hcl
+    KeyManager "hashicorp_vault" {
+        plugin_data {
+            vault_addr = "https://vault.example.org/"
+            pki_mount_point = "test-pki"
+            ca_cert_path = "/path/to/ca-cert.pem"
+            token_auth {
+               token = "<token>"
+            }
+            // If specify the token as an environment variable, set the empty structure.
+            // token_auth {}
+        }
+    }
+```
+
+## AppRole Authentication
+
+| key                      | type   | required | description                                                      | default                      |
+|:-------------------------|:-------|:---------|:-----------------------------------------------------------------|:-----------------------------|
+| approle_auth_mount_point | string |          | Name of the mount point where the AppRole auth method is mounted | approle                      |
+| approle_id               | string |          | An identifier of AppRole                                         | `${VAULT_APPROLE_ID}`        |
+| approle_secret_id        | string |          | A credential of AppRole                                          | `${VAULT_APPROLE_SECRET_ID}` |
+
+```hcl
+    KeyManager "hashicorp_vault" {
+        plugin_data {
+            vault_addr = "https://vault.example.org/"
+            pki_mount_point = "test-pki"
+            ca_cert_path = "/path/to/ca-cert.pem"
+            approle_auth {
+               approle_auth_mount_point = "my-approle-auth"
+               approle_id = "<Role ID>" // or specified by environment variables
+               approle_secret_id = "<Secret ID>" // or specified by environment variables
+            }
+            // If specify the approle_id and approle_secret as an environment variable and use the modified mount point
+            // approle_auth {
+            //    approle_auth_mount_point = "my-approle-auth"
+            // }
+
+            // If specify the approle_id and approle_secret as an environment variable and use the default mount point, set the empty structure.
+            // approle_auth {}
+        }
+    }
+```
+
+## Kubernetes Authentication
+
+| key                  | type   | required | description                                                                       | default    |
+|:---------------------|:-------|:---------|:----------------------------------------------------------------------------------|:-----------|
+| k8s_auth_mount_point | string |          | Name of the mount point where the Kubernetes auth method is mounted               | kubernetes |
+| k8s_auth_role_name   | string | ✔        | Name of the Vault role. The plugin authenticates against the named role           |            |
+| token_path           | string | ✔        | Path to the Kubernetes Service Account Token to use authentication with the Vault |            |
+
+```hcl
+    KeyManager "hashicorp_vault" {
+        plugin_data {
+            vault_addr = "https://vault.example.org/"
+            pki_mount_point = "test-pki"
+            ca_cert_path = "/path/to/ca-cert.pem"
+            k8s_auth {
+               k8s_auth_mount_point = "my-k8s-auth"
+               k8s_auth_role_name = "my-role"
+               token_path = "/path/to/sa-token"
+            }
+
+            // If specify role name and use the default mount point and token_path
+            // k8s_auth {
+            //   k8s_auth_role_name = "my-role"
+            // }            
+        }
+    }
+```