[Backport 2.x] Implement parallel execution of sub-queries for hybrid search (#749)

[VijayanB]

Description

Add new thread pool to schedule tasks that are related to hybrid query execution Register executor builders with Plugin Use Lucene's Task Executor to execute and collect results Parallelize Query re-write Parallelize score supplier creation Parallelize build hybrid scores

Backported #749 manually

Issues Resolved

Check List

• [x] New functionality includes testing.
  • [ ] All tests pass
• [ ] New functionality has been documented.
  • [ ] New functionality has javadoc added
• [x] Commits are signed as per the DCO using --signoff

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[martin-gaievski]

Please add [Backport 2.x] in PR header and commit ref in main from where we're cherry picking.

[VijayanB]

Please add [Backport 2.x] in PR header and commit ref in main from where we're cherry picking.

I added in commit. I will add in PR description as well. Thanks for feedback