Get list of headersToCopy from core and use getHeader(String headerName) instead of getHeaders()

[cwperks]

Description

Get list of headersToCopy from core and use getHeader(String headerName) instead of getHeaders()

This PR prevents creating a new map every time getHeaders() is called and only copies if any of the allowlisted headers are present on the request.

This PR will also fix compilation after associated PR in core was merged: opensearch-project/OpenSearch#19875

• Category (Enhancement, New feature, Bug fix, Test fix, Refactoring, Maintenance, Documentation)

Enhancement

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.

[cwperks]

Need new min distribution for 3.4.0.0-SNAPSHOT for the bwc tests

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.33%. Comparing base (02022f3) to head (a3f185c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...opensearch/security/filter/SecurityRestFilter.java	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5769      +/-   ##
==========================================
+ Coverage   73.30%   73.33%   +0.03%     
==========================================
  Files         435      435              
  Lines       26501    26509       +8     
  Branches     3949     3952       +3     
==========================================
+ Hits        19427    19441      +14     
+ Misses       5194     5185       -9     
- Partials     1880     1883       +3     

Files with missing lines	Coverage Δ
.../opensearch/security/OpenSearchSecurityPlugin.java	85.20% <100.00%> (ø)
...opensearch/security/filter/SecurityRestFilter.java	87.19% <93.33%> (+0.01%)	⬆️

... and 8 files with indirect coverage changes

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