[Backport 2.x] Improve GetAnomalyDetectorTransportAction to accept generic ActionRequest (#1150)

[ohltyler]

Description

Manual backport of #1150 due to conflict with imports. No logic changes.

Issues Resolved

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

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[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (b26c6c8) 79.73% compared to head (5532439) 79.80%.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #1151      +/-   ##
============================================
+ Coverage     79.73%   79.80%   +0.06%     
- Complexity     4357     4366       +9     
============================================
  Files           309      309              
  Lines         18170    18179       +9     
  Branches       1909     1909              
============================================
+ Hits          14488    14507      +19     
+ Misses         2772     2764       -8     
+ Partials        910      908       -2     

Flag	Coverage Δ
plugin	79.80% <22.22%> (+0.06%)	⬆️

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Files	Coverage Δ
...d/transport/GetAnomalyDetectorTransportAction.java	75.90% <100.00%> (+0.14%)	⬆️
...search/ad/transport/GetAnomalyDetectorRequest.java	84.31% <12.50%> (-13.37%)	⬇️

... and 6 files with indirect coverage changes