[Backport 2.x] Extracted the user attr handling methods from ConfigModelV7 into its own class

[opensearch-trigger-bot]

Backport 00c31e9 from #4416.

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.

Project coverage is 65.39%. Comparing base (932bf72) to head (2d2112d).
Report is 1 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x    #4431      +/-   ##
==========================================
+ Coverage   65.36%   65.39%   +0.03%     
==========================================
  Files         309      311       +2     
  Lines       21839    21867      +28     
  Branches     3547     3550       +3     
==========================================
+ Hits        14275    14301      +26     
+ Misses       5802     5800       -2     
- Partials     1762     1766       +4     

Files	Coverage Δ
...pensearch/security/securityconf/ConfigModelV7.java	67.68% <100.00%> (-0.48%)	⬇️
...opensearch/security/privileges/UserAttributes.java	76.00% <76.00%> (ø)

... and 6 files with indirect coverage changes