[BUG] High number of unnecessary computations in getIndices() of IndicesSegmentResponse

[gargharsh3134]

Describe the bug

For clusters with large number of shards and indices, the following computation is sub-optimal and leads to higher latencies for Rest APIs such as _cat/segments.

https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/action/admin/indices/segments/IndicesSegmentResponse.java#L96

Related component

Cluster Manager

To Reproduce

Create a cluster with high number of shards and indices (say 30k indices and 100k shards).
The latency of _cat/segments API will increase manifold.

Expected behavior

Refactoring using a map Map<String, List<ShardSegments>> indicesShardSegments = new HashMap<>(); will help save on extra computations and decrease latency of REST APIs.

Additional Details

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[dblock]

[Catch All Triage - 1, 2, 3, 4]

[sandeshkr419]

Thanks @gargharsh3134 for finding this and raising a draft PR as well. Please capture improvement in 1/ latency and 2/ heap usage so that we know the impact of this change.