[Backport 2.x] Manually backport: Introduce Identity Plugin to Core (#7246)

[stephen-crawford]

Manually backports #7246 to 2.x line.

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

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.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/14801/
• CommitID: da19ee6
  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?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/14802/
• CommitID: c5d6bf6
  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?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      2 org.opensearch.search.SearchWeightedRoutingIT.testStrictWeightedRoutingWithCustomString

• URL: https://build.ci.opensearch.org/job/gradle-check/14803/
• CommitID: 62947d0
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov-commenter]

Codecov Report

Merging #7393 (62947d0) into 2.x (d88c4a6) will decrease coverage by 0.13%.
The diff coverage is 52.02%.

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@@             Coverage Diff              @@
##                2.x    #7393      +/-   ##
============================================
- Coverage     70.36%   70.24%   -0.13%     
- Complexity    59753    59763      +10     
============================================
  Files          4850     4864      +14     
  Lines        287213   287384     +171     
  Branches      41727    41744      +17     
============================================
- Hits         202100   201867     -233     
- Misses        68217    68645     +428     
+ Partials      16896    16872      -24     

Impacted Files	Coverage Δ
...opensearch/identity/shiro/ShiroIdentityPlugin.java	0.00% <0.00%> (ø)
...pensearch/identity/shiro/ShiroSecurityManager.java	0.00% <0.00%> (ø)
...identity/shiro/UnsupportedAuthenticationToken.java	0.00% <0.00%> (ø)
...pensearch/common/settings/FeatureFlagSettings.java	50.00% <ø> (ø)
...opensearch/identity/tokens/RestTokenExtractor.java	0.00% <0.00%> (ø)
.../main/java/org/opensearch/rest/RestController.java	75.64% <4.76%> (-6.99%)	⬇️
...java/org/opensearch/identity/noop/NoopSubject.java	22.22% <22.22%> (ø)
...va/org/opensearch/identity/shiro/ShiroSubject.java	44.44% <44.44%> (ø)
server/src/main/java/org/opensearch/node/Node.java	84.84% <50.00%> (-0.30%)	⬇️
...n/java/org/opensearch/identity/NamedPrincipal.java	54.54% <54.54%> (ø)
... and 10 more

... and 501 files with indirect coverage changes