🚀 [Feature]: Adding support for logging in as a GitHub App (#126)

## Description

- Adding support for logging in as a GitHub App

## Type of change

<!-- Use the check-boxes [x] on the options that are relevant. -->

- [ ] 📖 [Docs]
- [ ] 🪲 [Fix]
- [ ] 🩹 [Patch]
- [ ] ⚠️ [Security fix]
- [x] 🚀 [Feature]
- [ ] 🌟 [Breaking change]

## Checklist

<!-- Use the check-boxes [x] on the options that are relevant. -->

- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas

.github/workflows/Linter.yml

@@ -30,3 +30,4 @@ jobs:
           VALIDATE_JSCPD: false
           VALIDATE_MARKDOWN_PRETTIER: false
           VALIDATE_YAML_PRETTIER: false
+          VALIDATE_GITLEAKS: false

src/functions/public/Apps/Get-GitHubApp.ps1

@@ -0,0 +1,35 @@
+filter Get-GitHubApp {
+    <#
+        .SYNOPSIS
+        Get the authenticated app
+
+        .DESCRIPTION
+        Returns the GitHub App associated with the authentication credentials used. To see how many app installations are associated with this
+        GitHub App, see the `installations_count` in the response. For more details about your app's installations, see the
+        "[List installations for the authenticated app](https://docs.github.com/rest/apps/apps#list-installations-for-the-authenticated-app)"
+        endpoint.
+
+        You must use a [JWT](https://docs.github.com/apps/building-github-apps/authenticating-with-github-apps/#authenticating-as-a-github-app)
+        to access this endpoint.
+
+        .EXAMPLE
+        Get-GitHubApp
+
+        Get the authenticated app.
+
+        .NOTES
+        [Get the authenticated app | GitHub Docs](https://docs.github.com/rest/apps/apps#get-the-authenticated-app)
+    #>
+    [OutputType([pscustomobject])]
+    [CmdletBinding()]
+    param ()
+
+    $inputObject = @{
+        APIEndpoint = '/app'
+        Method      = 'GET'
+    }
+
+    Invoke-GitHubAPI @inputObject | ForEach-Object {
+        Write-Output $_.Response
+    }
+}

src/functions/public/Apps/Get-GitHubAppJSONWebToken.ps1

@@ -0,0 +1,106 @@
+function Get-GitHubAppJSONWebToken {
+    <#
+        .SYNOPSIS
+        Generates a JSON Web Token (JWT) for a GitHub App.
+
+        .DESCRIPTION
+        Generates a JSON Web Token (JWT) for a GitHub App.
+
+        .EXAMPLE
+        Get-GitHubAppJWT -ClientId 'Iv987654321' -PrivateKeyFilePath '/path/to/private-key.pem'
+
+        Generates a JSON Web Token (JWT) for a GitHub App using the specified client ID and private key file path.
+
+        .EXAMPLE
+        Get-GitHubAppJWT -ClientId 'Iv987654321' -PrivateKey '--- BEGIN RSA PRIVATE KEY --- ... --- END RSA PRIVATE KEY ---'
+
+        Generates a JSON Web Token (JWT) for a GitHub App using the specified client ID and private key.
+
+        .OUTPUTS
+        System.String
+
+        .NOTES
+        [Generating a JSON Web Token (JWT) for a GitHub App | GitHub Docs](https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/generating-a-json-web-token-jwt-for-a-github-app#example-using-powershell-to-generate-a-jwt)
+    #>
+    [Diagnostics.CodeAnalysis.SuppressMessageAttribute(
+        'PSAvoidLongLines',
+        '',
+        Justification = 'Contains a long link.'
+    )]
+    [CmdletBinding(DefaultParameterSetName = 'PrivateKey')]
+    [Alias('Get-GitHubAppJWT')]
+    [OutputType([string])]
+    param(
+        # The client ID of the GitHub App.
+        # Can use the GitHub App ID or the client ID.
+        # Example: 'Iv23li8tyK9NUwl7rWlQ'
+        # Example: '123456'
+        [Parameter(Mandatory)]
+        [string] $ClientId,
+
+        # The path to the private key file of the GitHub App.
+        # Example: '/path/to/private-key.pem'
+        [Parameter(
+            Mandatory,
+            ParameterSetName = 'FilePath'
+        )]
+        [string] $PrivateKeyFilePath,
+
+        # The private key of the GitHub App.
+        # Example: @'
+        # -----BEGIN RSA PRIVATE KEY-----
+        # qwe
+        # ...
+        # -----END RSA PRIVATE KEY-----
+        # '@
+        [Parameter(
+            Mandatory,
+            ParameterSetName = 'PrivateKey'
+        )]
+        [string] $PrivateKey
+    )
+
+    if ($PrivateKeyFilePath) {
+        if (-not (Test-Path -Path $PrivateKeyFilePath)) {
+            throw "The private key path [$PrivateKeyFilePath] does not exist."
+        }
+
+        $PrivateKey = Get-Content -Path $PrivateKeyFilePath -Raw
+    }
+
+    $header = [Convert]::ToBase64String(
+        [System.Text.Encoding]::UTF8.GetBytes(
+            (
+                ConvertTo-Json -InputObject @{
+                    alg = 'RS256'
+                    typ = 'JWT'
+                }
+            )
+        )
+    ).TrimEnd('=').Replace('+', '-').Replace('/', '_')
+
+    $payload = [Convert]::ToBase64String(
+        [System.Text.Encoding]::UTF8.GetBytes(
+            (
+                ConvertTo-Json -InputObject @{
+                    iat = [System.DateTimeOffset]::UtcNow.AddSeconds(-10).ToUnixTimeSeconds()
+                    exp = [System.DateTimeOffset]::UtcNow.AddMinutes(10).ToUnixTimeSeconds()
+                    iss = $ClientId
+                }
+            )
+        )
+    ).TrimEnd('=').Replace('+', '-').Replace('/', '_')
+
+    $rsa = [System.Security.Cryptography.RSA]::Create()
+    $rsa.ImportFromPem($PrivateKey)
+
+    $signature = [Convert]::ToBase64String(
+        $rsa.SignData(
+            [System.Text.Encoding]::UTF8.GetBytes("$header.$payload"),
+            [System.Security.Cryptography.HashAlgorithmName]::SHA256,
+            [System.Security.Cryptography.RSASignaturePadding]::Pkcs1
+        )
+    ).TrimEnd('=').Replace('+', '-').Replace('/', '_')
+    $jwt = "$header.$payload.$signature"
+    $jwt
+}

tools/dev/Get-GitHubAppInstallations.ps1

@@ -0,0 +1,17 @@
+
+#List installations for the authenticated app
+Invoke-RestMethod -Uri 'https://api.github.com/app/installations' -Headers @{
+    Authorization = "Bearer $token"
+}
+
+
+#Get an organization installation for the authenticated app
+Invoke-RestMethod -Uri 'https://api.github.com/orgs/psmodule/installation' -Headers @{
+    Authorization = "Bearer $token"
+}
+
+
+#Get a repository installation for the authenticated app
+Invoke-RestMethod -Uri 'https://api.github.com/repos/psmodule/.github/installation' -Headers @{
+    Authorization = "Bearer $token"
+}