[2.x] Add an AD transport client

[ohltyler]

Description

Backport of #1110.

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

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[ohltyler]

Converting to draft until #1110 merged

[codecov]

Codecov Report

Merging #1111 (ea85bc2) into 2.x (558f556) will increase coverage by 0.17%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #1111      +/-   ##
============================================
+ Coverage     79.64%   79.81%   +0.17%     
- Complexity     4336     4353      +17     
============================================
  Files           307      309       +2     
  Lines         18159    18176      +17     
  Branches       1909     1909              
============================================
+ Hits          14462    14507      +45     
+ Misses         2786     2763      -23     
+ Partials        911      906       -5     

Flag	Coverage Δ
plugin	79.81% <100.00%> (+0.17%)	⬆️

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Files	Coverage Δ
...g/opensearch/ad/client/AnomalyDetectionClient.java	100.00% <100.00%> (ø)
...ensearch/ad/client/AnomalyDetectionNodeClient.java	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes