[Backport 1.x] Fixes custom action parsing

[opensearch-trigger-bot]

Backport 901d887 from #309

[codecov-commenter]

Codecov Report

Merging #310 (05415b0) into 1.x (15262d4) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                1.x     #310      +/-   ##
============================================
- Coverage     75.72%   75.71%   -0.01%     
  Complexity     2005     2005              
============================================
  Files           253      253              
  Lines         11467    11467              
  Branches       1815     1815              
============================================
- Hits           8683     8682       -1     
+ Misses         1760     1753       -7     
- Partials       1024     1032       +8     

Impacted Files	Coverage Δ
...anagement/indexstatemanagement/ISMActionsParser.kt	88.52% <100.00%> (+8.19%)	⬆️
...statemanagement/model/destination/CustomWebhook.kt	65.21% <0.00%> (-27.54%)	⬇️
...arch/indexmanagement/rollup/RollupSearchService.kt	55.55% <0.00%> (-3.71%)	⬇️
.../rollup/action/start/TransportStartRollupAction.kt	67.44% <0.00%> (-2.33%)	⬇️
...earch/indexmanagement/transform/model/Transform.kt	85.21% <0.00%> (-0.44%)	⬇️
...nt/indexstatemanagement/ManagedIndexCoordinator.kt	72.81% <0.00%> (-0.32%)	⬇️
.../opensearch/indexmanagement/rollup/model/Rollup.kt	84.90% <0.00%> (+0.47%)	⬆️
...ent/indexstatemanagement/util/ManagedIndexUtils.kt	78.30% <0.00%> (+1.41%)	⬆️
.../action/explain/TransportExplainTransformAction.kt	69.31% <0.00%> (+2.27%)	⬆️
...ch/indexmanagement/rollup/RollupMetadataService.kt	76.92% <0.00%> (+2.40%)	⬆️
... and 3 more

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[dblock]

@downsrob Is there an open issue on the flaky test?