[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

@@ -13,7 +13,6 @@ 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
 

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - [Writable Warm] Add composite directory implementation and integrate it with FileCache ([12782](https://github.com/opensearch-project/OpenSearch/pull/12782))
 - Fix race condition while parsing derived fields from search definition ([14445](https://github.com/opensearch-project/OpenSearch/pull/14445))
 - Add allowlist setting for ingest-common processors ([#14439](https://github.com/opensearch-project/OpenSearch/issues/14439))
+- Add new APIs in the Metadata to get different indices in the form of List ([#14557](https://github.com/opensearch-project/OpenSearch/pull/14557))
 
 ### Dependencies
 - Bump `org.gradle.test-retry` from 1.5.8 to 1.5.9 ([#13442](https://github.com/opensearch-project/OpenSearch/pull/13442))