[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.
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[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.13%. Comparing base (376fd16) to head (49dc4c0).
Report is 1 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   68.06%   68.13%   +0.07%     
==========================================
  Files         346      347       +1     
  Lines       24504    24544      +40     
  Branches     4062     4068       +6     
==========================================
+ Hits        16678    16723      +45     
+ Misses       5905     5898       -7     
- Partials     1921     1923       +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