[Feature Request] Enable Async Shard Batch Fetch By Default

[SwethaGuptha]

Is your feature request related to a problem? Please describe

Enable Async Shard Batch Fetch Feature by default for better scale and less memory pressure.

Creating the issue based on suggestion from @HenryTheSir on #17170 (comment)

Describe the solution you'd like

Set setting cluster.allocator.existing_shards_allocator.batch_enabled to true by default.

Related component

Cluster Manager

Describe alternatives you've considered

No response

Additional context

No response

[rajiv-kv]

[Triage - 1, 2]
Thanks for filing the issue.

We can also revisit the default values of related setting such as cluster.allocator.gateway.batch_size and timeout settings cluster.routing.allocation.shards_batch_gateway_allocator.* as part of enabling it by default.