[Backport 2.x] Implement new extension points in IdentityPlugin and add ContextProvidingPluginSubject

[cwperks]

Description

Backport #4896 to 2.x and target the new style authorization code path. A separate PR will be raised to implement this with the legacy authorization code path.

Also backports #4802 to resolve issues with codecov not reporting lines covered by tests in the integrationTest sourceSet.

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 82.71605% with 14 lines in your changes missing coverage. Please review.

Project coverage is 68.10%. Comparing base (4af1d07) to head (1ba3332).
Report is 1 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...ensearch/security/privileges/ActionPrivileges.java	59.09%	5 Missing and 4 partials ⚠️
.../opensearch/security/OpenSearchSecurityPlugin.java	83.33%	1 Missing ⚠️
.../org/opensearch/security/auth/UserSubjectImpl.java	90.00%	1 Missing ⚠️
...org/opensearch/security/filter/SecurityFilter.java	66.66%	0 Missing and 1 partial ⚠️
...ecurity/privileges/SystemIndexAccessEvaluator.java	92.85%	0 Missing and 1 partial ⚠️
...c/main/java/org/opensearch/security/user/User.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x    #5028      +/-   ##
==========================================
+ Coverage   64.44%   68.10%   +3.65%     
==========================================
  Files         344      346       +2     
  Lines       24433    24503      +70     
  Branches     4052     4061       +9     
==========================================
+ Hits        15747    16688     +941     
+ Misses       6749     5894     -855     
+ Partials     1937     1921      -16     

Files with missing lines	Coverage Δ
.../org/opensearch/security/auth/BackendRegistry.java	78.39% <100.00%> (+17.58%)	⬆️
...curity/identity/ContextProvidingPluginSubject.java	100.00% <100.00%> (ø)
...h/security/privileges/PrivilegesEvaluatorImpl.java	74.04% <100.00%> (+0.90%)	⬆️
...g/opensearch/security/support/ConfigConstants.java	95.23% <ø> (ø)
.../opensearch/security/OpenSearchSecurityPlugin.java	85.03% <83.33%> (+0.34%)	⬆️
.../org/opensearch/security/auth/UserSubjectImpl.java	90.00% <90.00%> (ø)
...org/opensearch/security/filter/SecurityFilter.java	66.50% <66.66%> (+0.15%)	⬆️
...ecurity/privileges/SystemIndexAccessEvaluator.java	76.92% <92.85%> (+5.60%)	⬆️
...c/main/java/org/opensearch/security/user/User.java	69.41% <0.00%> (+0.36%)	⬆️
...ensearch/security/privileges/ActionPrivileges.java	95.14% <59.09%> (+7.42%)	⬆️

... and 77 files with indirect coverage changes

[cwperks]

codecov is not picking up on lines hit through the integration tests for this change.

[cwperks]

Pushed a commit to backport #4802 in this PR as well.