[Backport 2.x] Moving ExtensionsRequest to use Protobuf

[saratvemulapalli]

Description

Manual backport of #6960 to 2.x

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.
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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/14400/
• CommitID: e1dea73
  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/14399/
• CommitID: dda5ddf
  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/14398/
• CommitID: 29b8a4b
  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?

[reta]

Apologies @saratvemulapalli , non-issue but I've just realized a minor update for protobuf is ongoing (https://github.com/opensearch-project/OpenSearch/pull/7261/files), not a big deal, I could rebase it after

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/14401/
• CommitID: b414608

[codecov-commenter]

Codecov Report

Merging #7265 (b414608) into 2.x (3f9845b) will decrease coverage by 0.04%.
The diff coverage is 64.28%.

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@@             Coverage Diff              @@
##                2.x    #7265      +/-   ##
============================================
- Coverage     70.46%   70.42%   -0.04%     
- Complexity    59796    59808      +12     
============================================
  Files          4847     4847              
  Lines        287150   287139      -11     
  Branches      41723    41723              
============================================
- Hits         202336   202219     -117     
- Misses        67891    68023     +132     
+ Partials      16923    16897      -26     

Impacted Files	Coverage Δ
...a/org/opensearch/extensions/ExtensionsManager.java	43.39% <0.00%> (-1.90%)	⬇️
...va/org/opensearch/extensions/ExtensionRequest.java	70.83% <69.23%> (-4.17%)	⬇️

... and 485 files with indirect coverage changes

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