Backport PR #15737 to 8.12: Fix logstash-keystore multiple keys operations with command flags

[github-actions]

Backport PR #15737 to 8.12 branch, original message:

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Release notes

[rn:skip]

What does this PR do?

Since logstash-keystore command supports multiple key (#15612), it has broken the following command in ECK as logstash-keystore takes --stdin - and any other possible flag - as the second key, and not as a command option:

echo 'y' | bin/logstash-keystore add HELLO --stdin < HELLO

This PR fixes how the keystore tool handle command's options, including validation for unknown options, and adding the --stdin flag to the add command.

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

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

How to test this PR locally

The following command should include a key named FOO with password FOO_PASSWORD:

> echo 'FOO_PASSWORD' > test.txt 
> echo 'y' | bin/logstash-keystore add FOO --stdin < test.txt

Related issues

• Closes: keystore command take --stdin as key #15724
• Relates: Add support to add and remove multiple keystore keys in a single operation #15612

[elastic-sonarqube]

Quality Gate passed

The SonarQube Quality Gate passed, but some issues were introduced.

3 New issues
0 Security Hotspots
96.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 517a7fb

[kaisecheng]

LGTM