[Backport 2.x] Manually backport #4632 (Allow multiple signing keys to be provided)

[stephen-crawford]

Description

This change manually backports #4632

We had to manually backport this change because there are some differences in the support of list.getFirst() between main and 2.x. The JWT classes are also slightly different between the two versions.

Testing

[Please provide details of testing done: unit testing, integration testing and manual 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

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[codecov]

Codecov Report

Attention: Patch coverage is 73.68421% with 10 lines in your changes missing coverage. Please review.

Project coverage is 65.40%. Comparing base (89ca6a8) to head (bb4a025).
Report is 2 commits behind head on 2.x.

Files	Patch %	Lines
...mazon/dlic/auth/http/jwt/HTTPJwtAuthenticator.java	72.22%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##              2.x    #4666   +/-   ##
=======================================
  Coverage   65.40%   65.40%           
=======================================
  Files         316      316           
  Lines       22142    22151    +9     
  Branches     3578     3581    +3     
=======================================
+ Hits        14482    14488    +6     
- Misses       5868     5869    +1     
- Partials     1792     1794    +2     

Files	Coverage Δ
...in/java/org/opensearch/security/util/KeyUtils.java	87.50% <100.00%> (-9.28%)	⬇️
...mazon/dlic/auth/http/jwt/HTTPJwtAuthenticator.java	70.68% <72.22%> (-0.87%)	⬇️

... and 4 files with indirect coverage changes

[DarshitChanpura]

Could you add a brief comment on why manual backport was required?