Add tooling for create sharded streams

[AbstractiveNord]

Proposed change

Add a feature 'sharded' for command 'stream add'. Idea is all about modification user subject by adding partition number into subject and internally create a requested count of stream-shards, also with subject mapping configuration for nats servers. For example, nats stream add streamname --shards=100.

Use case

Everytime when you want to split load or utilize more servers for storing data in Jetstream streams.

Contribution

Yes.