[Backport 2.x] Add extensions to gradle plugin

[ryanbogan]

Description

In order to run integration testing for the SDK and future extensions, the OpenSearch gradle plugin needs to be able to run both extensions and plugins at the same time. This PR adds the capability to run extensions from the gradle plugin, while leaving all existing plugin functionality untouched.

Backport of #7235

Issues Resolved

opensearch-project/opensearch-sdk-java#589

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

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/14904/
• CommitID: 23da2ae
  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
      1 org.opensearch.indices.replication.SegmentReplicationIT.testScrollWithOngoingSegmentReplication

• URL: https://build.ci.opensearch.org/job/gradle-check/14905/
• CommitID: 8707b17
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/14917/
• CommitID: 8707b17
  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: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/15046/
• CommitID: 8707b17

[codecov-commenter]

Codecov Report

Merging #7430 (8707b17) into 2.x (5e5d7ee) will increase coverage by 0.54%.
The diff coverage is 0.00%.

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@@             Coverage Diff              @@
##                2.x    #7430      +/-   ##
============================================
+ Coverage     70.34%   70.88%   +0.54%     
- Complexity    59728    60209     +481     
============================================
  Files          4863     4865       +2     
  Lines        287390   287453      +63     
  Branches      41749    41753       +4     
============================================
+ Hits         202161   203775    +1614     
+ Misses        68230    67000    -1230     
+ Partials      16999    16678     -321     

Impacted Files	Coverage Δ
...arch/gradle/testclusters/ExtensionsProperties.java	0.00% <0.00%> (ø)
...nsearch/gradle/testclusters/OpenSearchCluster.java	0.00% <0.00%> (ø)
...opensearch/gradle/testclusters/OpenSearchNode.java	0.00% <0.00%> (ø)
.../gradle/testclusters/TestClusterConfiguration.java	0.00% <ø> (ø)
...search/gradle/testclusters/TestExtensionsList.java	0.00% <0.00%> (ø)

... and 524 files with indirect coverage changes