[Backport 2.x] Adding field level security test cases for FlatFields

[opensearch-trigger-bot]

Backport b4413a7 from #2876

[cwperks]

@peternied The backports needs to be modified to account for Apache HttpClient4 in the 2.x line instead of Apache HttpClient5 which is in main.

[codecov]

Codecov Report

Merging #2879 (2426102) into 2.x (2426102) will not change coverage.
The diff coverage is n/a.

❗ Current head 2426102 differs from pull request most recent head 1d53133. Consider uploading reports for the commit 1d53133 to get more accurate results

@@            Coverage Diff            @@
##                2.x    #2879   +/-   ##
=========================================
  Coverage     61.49%   61.49%           
  Complexity     3381     3381           
=========================================
  Files           264      264           
  Lines         18645    18645           
  Branches       3287     3287           
=========================================
  Hits          11465    11465           
  Misses         5597     5597           
  Partials       1583     1583           

[cwperks]

@mingshl These backports need to be updated to account for Apache HttpClient4 on 2.x instead of HttpClient5 on main. I can help update the backports tomorrow if needed.

[cwperks]

Closing in favor of: #2893