[Backport 2.7] Add support of .opensearch-knn-model as system index to transport actions

[opensearch-trigger-bot]

Backport 1b6fd48 from #847

[codecov-commenter]

Codecov Report

Merging #849 (4d5f849) into 2.7 (e6c6267) will decrease coverage by 0.02%.
The diff coverage is 89.61%.

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@@             Coverage Diff              @@
##                2.7     #849      +/-   ##
============================================
- Coverage     85.21%   85.20%   -0.02%     
- Complexity     1085     1090       +5     
============================================
  Files           151      152       +1     
  Lines          4404     4414      +10     
  Branches        393      396       +3     
============================================
+ Hits           3753     3761       +8     
  Misses          474      474              
- Partials        177      179       +2     

Impacted Files	Coverage Δ
...n/plugin/transport/SearchModelTransportAction.java	81.81% <71.42%> (+6.81%)	⬆️
...plugin/transport/TrainingModelTransportAction.java	86.11% <81.81%> (+1.26%)	⬆️
...org/opensearch/knn/common/ThreadContextHelper.java	83.33% <83.33%> (ø)
...main/java/org/opensearch/knn/indices/ModelDao.java	81.55% <91.89%> (-1.12%)	⬇️
...n/plugin/transport/DeleteModelTransportAction.java	100.00% <100.00%> (ø)
.../knn/plugin/transport/GetModelTransportAction.java	100.00% <100.00%> (ø)
...in/transport/TrainingJobRouterTransportAction.java	80.85% <100.00%> (+0.85%)	⬆️

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