Make HASHED_PREFIX as the default path type for remote store and snapshot

[ashking94]

Description

1. The default value for cluster.remote_store.index.path.type has been changed from FIXED to HASHED_PREFIX. This setting determines the path type used for data stored in the remote store. The HASHED_PREFIX option provides better distribution of keys and has been known to scale better with growing data size.

2. The default value for shard_path_type has been changed from FIXED to HASHED_PREFIX. This setting determines the path type used for shard files in the blob store repository. Similar to the remote store settings, the HASHED_PREFIX option provides better distribution of shard files.

These changes are intended to improve the default performance and reliability of remote store and snapshot functionality in OpenSearch.

Let me know if you would like me to modify or expand on anything in this PR description.

Check List

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

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❌ Gradle check result for cd69552: FAILURE

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❌ Gradle check result for a304b4c: FAILURE

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❌ Gradle check result for ed4a9c7: FAILURE

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[github-actions]

✅ Gradle check result for a2898c3: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.55%. Comparing base (a6724d3) to head (a2898c3).
Report is 12 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #18163      +/-   ##
============================================
+ Coverage     72.53%   72.55%   +0.01%     
- Complexity    67207    67232      +25     
============================================
  Files          5476     5476              
  Lines        310436   310438       +2     
  Branches      45121    45121              
============================================
+ Hits         225179   225237      +58     
+ Misses        66894    66889       -5     
+ Partials      18363    18312      -51     

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[andrross]

@ashking94 We disabled using HASHED_PREFIX in #17996 because it was causing test failures. Did we get to the bottom of that? There might still be an underlying bug there.