Enable testing for ExtensiblePlugins using classpath plugins

[cwperks]

Description

This PR enables testing for ExtensiblePlugins using the classpath plugin pattern present in internalClusterTest.

To do this, this PR tries to load all extensions for classpath plugins by defining an extendedPlugin relationship between a classpath plugin and all other classpath plugins. In normal scenarios, a plugin will declare that it extends another plugin through the extendedPlugins keyword that it places in build.gradle (example). This PR also puts logic in place to ensure that any extensions are only loaded once.

Additional Context

I ran into this issue while using the integration test framework in the security plugin to test out a change I have been experimenting with. The integration test framework utilizes classpath plugins and it was impossible to load an extensible plugin and its extension simultaneously and have the extensions loaded.

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 4176c46: FAILURE

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 result for 4176c46: FAILURE

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 result for ef041b0: UNSTABLE

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]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.15%. Comparing base (38a4112) to head (ef041b0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...in/java/org/opensearch/plugins/PluginsService.java	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16908      +/-   ##
============================================
- Coverage     72.25%   72.15%   -0.10%     
+ Complexity    65224    65216       -8     
============================================
  Files          5297     5297              
  Lines        303324   303330       +6     
  Branches      43913    43915       +2     
============================================
- Hits         219161   218878     -283     
- Misses        66213    66530     +317     
+ Partials      17950    17922      -28     

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