[Searchable Snapshots] Refactor BalancedShardsAllocator.Balancer for managing local shards

[kotwanikunal]

Is your feature request related to a problem? Please describe.
Currently, OpenSearch allocation and initialization works solely for shards which are located on the node’s filesystem. The allocation process should take into account the new remote shards while assigning, moving or balancing them across clusters such that they are only assigned to nodes with the specific search role.

Describe the solution you'd like

• Start with abstracting out the Balancer into an abstract ShardsBalancer class which can provide a blueprint for the upcoming RemoteShardsBalancer
• Refactor the BalancedShardsAllocator.Balancer class to LocalShardsBalancer
• Details: [Searchable Snapshot] [Design Proposal] Shard Allocator for Remote Search Shards #4759

Describe alternatives you've considered

• Details: [Searchable Snapshot] [Design Proposal] Shard Allocator for Remote Search Shards #4759

Additional context
NA