Interim build fix for PluginSubject related changes

[cwperks]

Description

This PR fixes compilation issues due to a change in the IdentityPlugin interface introduced in opensearch-project/OpenSearch#14630

There is a PR open on this repo to implement the extension points as intended, but the PR will need some time for thorough review.

This PR is an interim change to ensure that the plugin compiles. The code is not currently used and guarded by the identity feature flag: opensearch.experimental.feature.identity.enabled

The PluginSubject being introduced in this PR is a noop and simply stashes the ThreadContext.

• Category (Enhancement, New feature, Bug fix, Test fix, Refactoring, Maintenance, Documentation)

Enhancement

Issues Resolved

Related to: #4439

Is this a backport? If so, please add backport PR # and/or commits #, and remove backport-failed label from the original PR.

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• API changes companion pull request created
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 65.23%. Comparing base (0b66f48) to head (2715e0b).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...pensearch/security/identity/NoopPluginSubject.java	0.00%	6 Missing ⚠️
.../opensearch/security/OpenSearchSecurityPlugin.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4690      +/-   ##
==========================================
- Coverage   65.26%   65.23%   -0.03%     
==========================================
  Files         317      318       +1     
  Lines       22320    22327       +7     
  Branches     3591     3591              
==========================================
- Hits        14568    14566       -2     
- Misses       5953     5964      +11     
+ Partials     1799     1797       -2     

Files with missing lines	Coverage Δ
.../opensearch/security/OpenSearchSecurityPlugin.java	84.47% <0.00%> (-0.12%)	⬇️
...pensearch/security/identity/NoopPluginSubject.java	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

[cwperks]

CI checks will pass after min distribution is updated