Proposed change

Add a helper method to the jetstream::Context struct to allow getting or creating a KV store in one call. This allows both Jetstreams and KVs to be created in code.

Use case

As a user I would like to ensure a KV exists and if not is created in one call before populating it.

Contribution

I have my changes ready in a branch : https://github.com/Callum-A/nats.rs/tree/feat/get-or-create-kv