[DRAFT] Introduce PluginMetadataClient

[fddattal]

Description

[Describe what this change achieves]

This PR introduces the PluginMetadataClient interface which abstractly exposes core metadata operations to plugins.

This pr is currently in draft. Tests are pending.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Reference RFC: #13274

Check List

• [TODO] New functionality includes testing.
  • [TODO] All tests pass
• [Y] New functionality has been documented.
  • [Y] New functionality has javadoc added
• [N/A] API changes companion pull request created.
• [TODO] Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• [TODO] Commits are signed per the DCO using --signoff
• [TODO] Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• [TODO] Public documentation issue/PR created

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[github-actions]

❌ Gradle check result for 9d9e2dd: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[dblock]

Looking at this code, is the SdkAwarePlugin additional abstraction really necessary? What's the reason that it's not called MetadataPlugin instead?