FTS - forceMerge api to trigger compactions on replica partitions

The force merge/compaction operation isn't triggering compactions on the replica partitions. It only acts on the primary partitions of an index.

Let's investigate and gain more insights into the customer use cases to start supporting force merge on replica partitions.

On a higher level, the force merge operation primarily helps in 

1 - reclaiming the disk space with an erratic/heavy update/delete driven use case on an existing index.  (retain the index size within limits)

2 - it would also help in improving the latency/throughput for an index with a lesser number of segment files at the root.

So, performing forced compactions on the replica partitions would still help to reclaim the disk space faster, but at the increased cost of resources for performing the compactions.

While creating this ticket, search don't leverage replica partitions for serving query.