[Backport 2.x] Switching fieldcaps api to utilize js client

[opensearch-trigger-bot]

Backport 05c55bf from #984

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.74%. Comparing base (a6019e4) to head (be44fe4).
Report is 23 commits behind head on 2.x.

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x     #994      +/-   ##
==========================================
+ Coverage   52.36%   53.74%   +1.38%     
==========================================
  Files         168      170       +2     
  Lines        6541     6955     +414     
  Branches     1336     1430      +94     
==========================================
+ Hits         3425     3738     +313     
- Misses       2718     2787      +69     
- Partials      398      430      +32     

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