feat(config): support loading secrets from files and directories

[tie]

This change adds a secrets backend that retrieves secrets from files and directories. For example, this is useful for loading secrets from systemd credentials directory and similar mechanisms.
E.g.

[secret.systemd_credentials]
type = "directory"
path = "$CREDENTIALS_DIRECTORY"
remove_trailing_whitespace = true

SECRET[systemd_credentials.password.txt] → $CREDENTIALS_DIRECTORY/password.txt

Closes #20747

References:

• Validation fails with --no-environment flag if environment variables are not defined #21270
• feat(http_client): Add token_file feature #20138 (review)

[bits-bot]

All committers have signed the CLA.

[jszwedko]

Apologies @tie ! This one fell off my radar. I found it again skimming open PRs. This looks like a great addition. I appreciate you adding the behavior tests.

[jszwedko]

Could you add a changelog fragment here? I attempted to, but I'm not able to push to this branch. You can apply this diff:

ffbf204849 - (HEAD -> tie/master) add changelog entry (32 seconds ago) <Jesse Szwedko>
diff --git a/changelog.d/add-file-secrets.enhancement.md b/changelog.d/add-file-secrets.enhancement.md
new file mode 100644
index 0000000000..6c80fe8540
--- /dev/null
+++ b/changelog.d/add-file-secrets.enhancement.md
@@ -0,0 +1,4 @@
+Vector now supports two additional back-ends for loading secrets: `file`, for leading a set of
+secrets from a JSON file, and `directory`, for loading secrets from a list of files.
+
+authors: tie

[tie]

@jszwedko, done.

[jszwedko]

Thank you!