Make user attribute serialization property dynamic

[cwperks]

Description

Re-creating #5657 with a test fix.

This PR makes the configuration setting for plugins.security.user_attribute_serialization.enableddynamic, so it can be updated via requests to PUT cluster/_settings.

• Category - Enhancement
• Why these changes are required? - Makes the configuration of this setting more dynamic, which is especially useful in testing contexts
• What is the old behavior before changes and new behavior after changes? - Before: The setting could only be configured statically in opensearch.yml. After: The setting can be dynamically updated with requests to PUT cluster/_settings.

Issues Resolved

Is this a backport? If so, please add backport PR # and/or commits #, and remove backport-failed label from the original PR.

Do these changes introduce new permission(s) to be displayed in the static dropdown on the front-end? If so, please open a draft PR in the security dashboards plugin and link the draft PR here

Testing

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• API changes companion pull request created
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[DarshitChanpura]

Changes LGTM. left one comment. Will approve once addressed

[codecov]

Codecov Report

❌ Patch coverage is 58.82353% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.93%. Comparing base (3274f9d) to head (8290d4f).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
...arch/security/ssl/OpenSearchSecuritySSLPlugin.java	0.00%	12 Missing ⚠️
...earch/security/privileges/PrivilegesEvaluator.java	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5673      +/-   ##
==========================================
- Coverage   72.97%   72.93%   -0.04%     
==========================================
  Files         414      414              
  Lines       25873    25902      +29     
  Branches     3934     3934              
==========================================
+ Hits        18881    18892      +11     
- Misses       5079     5098      +19     
+ Partials     1913     1912       -1     

Files with missing lines	Coverage Δ
.../opensearch/security/OpenSearchSecurityPlugin.java	85.41% <100.00%> (+0.17%)	⬆️
.../opensearch/security/support/SecuritySettings.java	80.00% <100.00%> (+5.00%)	⬆️
...earch/security/privileges/PrivilegesEvaluator.java	78.86% <75.00%> (-0.15%)	⬇️
...arch/security/ssl/OpenSearchSecuritySSLPlugin.java	83.66% <0.00%> (-3.84%)	⬇️

... and 5 files with indirect coverage changes

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