[Metadata Immutability] Change different indices lookup objects from array type to lists

Changed the arrays to immutable List instances, added new versions of the getters which returns List instances.

Resolves #8647

Signed-off-by: Abdul Muneer Kolarkunnu <[email protected]>

Author: akolarkunnu

CHANGELOG-3.0.md

@@ -13,6 +13,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - GHA to verify checklist items completion in PR descriptions ([#10800](https://github.com/opensearch-project/OpenSearch/pull/10800))
 - Allow to pass the list settings through environment variables (like [], ["a", "b", "c"], ...) ([#10625](https://github.com/opensearch-project/OpenSearch/pull/10625))
 - Views, simplify data access and manipulation by providing a virtual layer over one or more indices ([#11957](https://github.com/opensearch-project/OpenSearch/pull/11957))
+- Added new APIs in the Metadata to get different indices in the form of List ([#14557](https://github.com/opensearch-project/OpenSearch/pull/14557))
 
 ### Dependencies