[Backport 2.17] pluggable deciders for concurrent search (#15363)

[opensearch-trigger-bot]

Backport 39aa3e4 from #15630.

[github-actions]

✅ Gradle check result for 7e48ed1: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 22 lines in your changes missing coverage. Please review.

Project coverage is 71.76%. Comparing base (d000171) to head (7e48ed1).
Report is 1 commits behind head on 2.17.

Files with missing lines	Patch %	Lines
...va/org/opensearch/search/DefaultSearchContext.java	76.47%	4 Missing and 8 partials ⚠️
...arch/search/deciders/ConcurrentSearchDecision.java	86.95%	2 Missing and 1 partial ⚠️
.../main/java/org/opensearch/index/IndexSettings.java	60.00%	1 Missing and 1 partial ⚠️
...main/java/org/opensearch/search/SearchService.java	66.66%	1 Missing and 1 partial ⚠️
...search/common/settings/AbstractScopedSettings.java	93.33%	0 Missing and 1 partial ⚠️
...earch/search/deciders/ConcurrentSearchDecider.java	0.00%	1 Missing ⚠️
...earch/search/deciders/ConcurrentSearchVisitor.java	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               2.17   #15635      +/-   ##
============================================
- Coverage     72.11%   71.76%   -0.36%     
+ Complexity    64396    64113     -283     
============================================
  Files          5244     5247       +3     
  Lines        299367   299480     +113     
  Branches      43575    43603      +28     
============================================
- Hits         215901   214916     -985     
- Misses        65723    66584     +861     
- Partials      17743    17980     +237     

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