Implement a POC based on BalancedShardsAllocator for searchable snapshots

[kotwanikunal]

Is your feature request related to a problem? Please describe.

• Searchable snapshots will introduce a new remote searcher role - [Searchable Snapshot] [Design Proposal] Remote Searchable Snapshots #3895
• As a part of this, we would like to design a new shard allocation logic which will assign the searchable snapshot shards to the nodes with this new role

Describe the solution you'd like
This task will focus on the following -

• Implement a POC using the existing BalancedShardsAllocator logic

Describe alternatives you've considered

• N/A

Additional context

• [Searchable Snapshot] [Spike] Shard allocation for searchable snapshots  #4152
• [Searchable Snapshot] Shard allocation for searchable snapshots #4306

[kotwanikunal]

POC at kotwanikunal#4