[2.x] Implement new extension points in IdentityPlugin and add ContextProvidingPluginSubject - legacy authz code path

[cwperks]

Description

This PR implements the legacy authz code path for #4665

Companion PR in core: opensearch-project/OpenSearch#14630

This PR by itself does not add additional functionality, it simply implements the new extension points in core and introduces a new class called ContextProvidingPluginSubject which populates a header in the ThreadContext with the canonical class name of the plugin that is executing code using pluginSystemSubject.runAs(() -> { ... }). See ./gradlew integrationTest --tests SystemIndexTests for an example that verifies that a block of code run with pluginSystemSubject.runAs(() -> { ... }) has contextual info in the thread context.

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

Enhancement

Issues Resolved

Related to #4439

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 94.11765% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.15%. Comparing base (d11348a) to head (a2cebf2).
Report is 2 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...search/security/transport/SecurityInterceptor.java	60.00%	1 Missing and 1 partial ⚠️
.../privileges/legacy/SystemIndexAccessEvaluator.java	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x    #5037      +/-   ##
==========================================
+ Coverage   68.04%   68.15%   +0.10%     
==========================================
  Files         346      347       +1     
  Lines       24504    24544      +40     
  Branches     4062     4068       +6     
==========================================
+ Hits        16674    16727      +53     
+ Misses       5906     5895      -11     
+ Partials     1924     1922       -2     

Files with missing lines	Coverage Δ
...ecurity/privileges/SystemIndexAccessEvaluator.java	76.92% <100.00%> (ø)
...ity/privileges/legacy/PrivilegesEvaluatorImpl.java	62.73% <100.00%> (+1.31%)	⬆️
...pensearch/security/securityconf/ConfigModelV7.java	67.92% <100.00%> (+0.37%)	⬆️
...security/securityconf/InMemorySecurityRolesV7.java	100.00% <100.00%> (ø)
...rch/security/transport/SecurityRequestHandler.java	55.03% <100.00%> (ø)
.../privileges/legacy/SystemIndexAccessEvaluator.java	72.85% <92.85%> (+2.22%)	⬆️
...search/security/transport/SecurityInterceptor.java	80.00% <60.00%> (-0.69%)	⬇️

... and 7 files with indirect coverage changes

[derek-ho]

Few minor comments, but lgtm mostly