Add SplitResponseProcessor to allowlist

[dbwiddis]

Description

The SplitResponseProcessor for Search Pipelines was correctly added here:
https://github.com/opensearch-project/OpenSearch/pull/14800/files#diff-13c9de4d1b7eaef9dac008ddbff90b83b84e00dc13c2547c6c67484f26efc1e2

However, an incorrect (possibly automated) merge conflict resolution inadvertently removed it from the allowlist (and associated tests) here:
https://github.com/opensearch-project/OpenSearch/pull/14785/files#diff-13c9de4d1b7eaef9dac008ddbff90b83b84e00dc13c2547c6c67484f26efc1e2

This PR puts it back.

Related Issues

Resolves #15229

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable. Add tests for SplitResponseProcessor opensearch-api-specification#526
• Public documentation issue/PR created, if applicable. [DOC] Split Response Processor is not available in 2.16 documentation-website#8077

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.

[owaiskazi19]

LGTM

[github-actions]

✅ Gradle check result for 1672e61: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.89%. Comparing base (0eb63bd) to head (1672e61).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #15393      +/-   ##
============================================
- Coverage     71.90%   71.89%   -0.01%     
- Complexity    63244    63261      +17     
============================================
  Files          5224     5224              
  Lines        296077   296077              
  Branches      42762    42762              
============================================
- Hits         212884   212859      -25     
- Misses        65661    65742      +81     
+ Partials      17532    17476      -56     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[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:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-15393-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 2301adf9a0e2921d2fa359c51ee5c87309ca71ad
# Push it to GitHub
git push --set-upstream origin backport/backport-15393-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-15393-to-2.x.

[dblock]

Will need a manual backport please @dbwiddis .