Settings Backwards Compatibility

[bowenlan-amzn]

Issue #, if available:
#11
Description of changes:

Cluster setting is updated to legacy setting because there is clusterService.clusterSettings.addSettingsUpdateConsumer
As DB mentioned here, this consumer can update the setting to legacy setting when OpenSearch node joins the cluster

But for index setting, there is no such consumer. So we will need read from 2 places (_plugins first, if not exist, read _opendistro one); and only write to _opendistro. This will not break customer's existing use case.

CheckList:
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[codecov-commenter]

Codecov Report

Merging #16 (18ffcf6) into main (eee3191) will increase coverage by 0.73%.
The diff coverage is 97.17%.

@@             Coverage Diff              @@
##               main      #16      +/-   ##
============================================
+ Coverage     76.84%   77.57%   +0.72%     
- Complexity     1559     1560       +1     
============================================
  Files           199      201       +2     
  Lines          8395     8588     +193     
  Branches       1323     1326       +3     
============================================
+ Hits           6451     6662     +211     
+ Misses         1246     1224      -22     
- Partials        698      702       +4     

Impacted Files	Coverage Δ	Complexity Δ
...arch/indexmanagement/rollup/RollupMapperService.kt	58.87% <0.00%> (-0.39%)	23.00 <0.00> (-1.00)
...temanagement/opensearchapi/OpenSearchExtensions.kt	80.95% <50.00%> (-0.53%)	0.00 <0.00> (ø)
...nsearch/indexmanagement/rollup/util/RollupUtils.kt	80.08% <60.00%> (+0.82%)	0.00 <0.00> (ø)
.../rollup/settings/LegacyOpenDistroRollupSettings.kt	97.95% <97.95%> (ø)	1.00 <1.00> (?)
...t/settings/LegacyOpenDistroManagedIndexSettings.kt	99.09% <99.09%> (ø)	1.00 <1.00> (?)
...pensearch/indexmanagement/IndexManagementPlugin.kt	90.79% <100.00%> (+1.66%)	12.00 <1.00> (ø)
...exstatemanagement/settings/ManagedIndexSettings.kt	98.94% <100.00%> (+0.02%)	1.00 <0.00> (ø)
...xmanagement/rollup/actionfilter/FieldCapsFilter.kt	46.54% <100.00%> (ø)	21.00 <0.00> (ø)
...management/rollup/interceptor/RollupInterceptor.kt	68.96% <100.00%> (ø)	31.00 <0.00> (ø)
.../indexmanagement/rollup/settings/RollupSettings.kt	97.56% <100.00%> (ø)	1.00 <0.00> (ø)
... and 8 more

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