Add StoreFactory plugin interface for custom Store implementations

[xuxiong1]

Description

This enables plugins to provide custom Store implementations per index.

• Extend IndexStorePlugin interface with StoreFactory and getStoreFactories() method
• Add INDEX_STORE_FACTORY_SETTING to allow plugins to provide custom Store factories
• Update IndexService to use pluggable StoreFactory for shard Store creation
• Make Store.LoadedMetadata and Store.OnClose public API for custom store plugin developers
• Support factory selection via index.store.factory setting with fallback to default Store

Will have a PR later to actually implement the Store plugin, which would have a custom store implementation to handle files outside the default store index directory.

Related Issues

Resolves #19090

Check List

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

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[xuxiong1]

@yupeng9 @msfroh, please take a look, thanks!

[github-actions]

✅ Gradle check result for 01750d3: SUCCESS

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.86%. Comparing base (1273c5a) to head (f56b79c).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/main/java/org/opensearch/index/IndexModule.java	91.66%	0 Missing and 1 partial ⚠️
.../java/org/opensearch/plugins/IndexStorePlugin.java	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #19091      +/-   ##
============================================
- Coverage     72.93%   72.86%   -0.08%     
- Complexity    69411    69420       +9     
============================================
  Files          5647     5647              
  Lines        319139   319161      +22     
  Branches      46163    46165       +2     
============================================
- Hits         232775   232544     -231     
- Misses        67502    67812     +310     
+ Partials      18862    18805      -57     

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[github-actions]

✅ Gradle check result for f56b79c: SUCCESS