[8.17] (backport #17351) Improve the key validation in secret identifier.

[mergify]

Release notes

• The keystore has improved validation, and will no longer allow the creation of secrets that cannot be accessed with config replacement parameters (${ ... }).

What does this PR do?

Before this change, key-value can be added to keystore but it might impossible to reference the key because ConfigVariableExpander ignores if key name doesn't align with SUBSTITUTION_PLACEHOLDER_REGEX
Improves user experience with the secret store CLI that when providing key name, LS validates to accept a value which can be used in pipeline (aligns with ConfigVariableExpander#SUBSTITUTION_PLACEHOLDER_REGEX).
If already invalid key(s) were added, it warns to remove/replace the keys.

Why is it important/What is the impact to the user?

No user impact. Keystore CLI restricts adding meaningless keys to keystore.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• [] I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

Related issues

Use cases

Screenshots

Logs

---

This is an automatic backport of pull request #17351 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of d24675a has failed:

On branch mergify/bp/8.17/pr-17351
Your branch is up to date with 'origin/8.17'.

You are currently cherry-picking commit d24675ae.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   logstash-core/src/main/java/org/logstash/plugins/ConfigVariableExpander.java
	modified:   logstash-core/src/main/java/org/logstash/secret/SecretIdentifier.java
	modified:   logstash-core/src/main/java/org/logstash/secret/cli/SecretStoreCli.java
	modified:   logstash-core/src/test/java/org/logstash/secret/cli/SecretStoreCliTest.java

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/reference/keystore.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

📃 DOCS PREVIEW ✨ https://logstash_bk_17585.docs-preview.app.elstc.co/diff

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: e162655

cc @mashhurs

[mashhurs]

Rendered doc:

[mashhurs]

lgtm!