[Searchable Snapshot] Shard allocation 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

• As a part of [Searchable Snapshot] [Spike] Shard allocation for searchable snapshots  #4152, we have worked on understanding the current allocation and routing methodology
• This task will focus on the following -
  • Implement a POC using the existing BalancedShardsAllocator logic
  • Design a new ShardsAllocator implementation which will account for the pre-existing shards on the cluster and the shards from searchable snapshots.

Describe alternatives you've considered

• N/A

Additional context
Add any other context or screenshots about the feature request here.

[andrross]

Closing this issue and will track the next level of design work in #4535