[Tiered Caching] Remove PLUGGABLE_CACHE feature flag

[peteralfonsi]

Description

As tiered caching will no longer be experimental in 3.0, removes its feature flag. Now users can always specify a store name via setting, and cache implementations will always track their own stats as exposed in GET _nodes/stats/caches/....

Modifies some UTs to account for this. Manually tested with debugger that the TSC can be used in the IRC via setting with the new changes. Also manually checked the response in GET _nodes/stats/caches/request_cache is populated even for the default cache.

Related Issues

Resolves #17343
Sub-issue of #17303

Check List

• Functionality includes testing.
• [N/A] API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[peteralfonsi]

Flaky test: #17291

[github-actions]

❕ Gradle check result for 8a559cb: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.43%. Comparing base (7388205) to head (6092df8).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
.../opensearch/common/cache/service/CacheService.java	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #17344      +/-   ##
============================================
+ Coverage     71.54%   72.43%   +0.89%     
- Complexity    64883    65749     +866     
============================================
  Files          5311     5311              
  Lines        304903   304898       -5     
  Branches      44215    44213       -2     
============================================
+ Hits         218133   220865    +2732     
+ Misses        68896    66016    -2880     
- Partials      17874    18017     +143     

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[github-actions]

❌ Gradle check result for de67767: FAILURE

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?

[peteralfonsi]

Flaky test: #17540

[github-actions]

❕ Gradle check result for 6092df8: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.snapshots.DedicatedClusterSnapshotRestoreIT.testSnapshotWithStuckNode

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[peteralfonsi]

@andrross Could you take another look at this one when you get a chance?