Add feature flag for extensions

[ryanbogan]

Signed-off-by: Ryan Bogan [email protected]

Description

Adds a feature flag for the functionality of extensions.

Issues Resolved

3851

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: UNSTABLE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/6792/
• CommitID: 827488d
  Please examine the workflow log, locate, and copy-paste the failure 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/6793/
• CommitID: e8f7b69

[codecov-commenter]

Codecov Report

Merging #5211 (e8f7b69) into main (4fc5c8d) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #5211      +/-   ##
============================================
- Coverage     71.00%   70.90%   -0.11%     
+ Complexity    58168    58117      -51     
============================================
  Files          4708     4708              
  Lines        277565   277565              
  Branches      40190    40190              
============================================
- Hits         197094   196805     -289     
- Misses        64310    64614     +304     
+ Partials      16161    16146      -15     

Impacted Files	Coverage Δ
.../java/org/opensearch/common/util/FeatureFlags.java	50.00% <ø> (ø)
...adonly/AddIndexBlockClusterStateUpdateRequest.java	0.00% <0.00%> (-75.00%)	⬇️
...a/org/opensearch/client/cluster/SniffModeInfo.java	0.00% <0.00%> (-58.83%)	⬇️
...readonly/TransportVerifyShardIndexBlockAction.java	9.75% <0.00%> (-58.54%)	⬇️
...n/admin/indices/readonly/AddIndexBlockRequest.java	17.85% <0.00%> (-53.58%)	⬇️
...pensearch/indices/breaker/CircuitBreakerStats.java	27.77% <0.00%> (-41.67%)	⬇️
...indices/readonly/TransportAddIndexBlockAction.java	20.68% <0.00%> (-41.38%)	⬇️
...pensearch/action/ingest/DeletePipelineRequest.java	31.25% <0.00%> (-37.50%)	⬇️
.../admin/indices/readonly/AddIndexBlockResponse.java	0.00% <0.00%> (-37.50%)	⬇️
.../opensearch/client/indices/CloseIndexResponse.java	42.50% <0.00%> (-35.00%)	⬇️
... and 479 more

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-5211-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 5ad8252246a599baa0477927efd59ce366b3b9ce
# Push it to GitHub
git push --set-upstream origin backport/backport-5211-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-5211-to-2.x.