[watermill-nats] new jetstream API

[AlexCuse]

Add a new pull subscribe option and target separately using eg https://natsbyexample.com/examples/jetstream/api-migration/go

I expect existing callback-based implementations will continue to work at least until v2 of the nats.go client is out. But batching pull subscribers should achieve better throughput in scenarios where order isn't terribly important.

[AlexCuse]

There are some changing semantics here. Eg queue groups are achieved through multiple pull subscriptions to the same broker-managed consumer. Maybe implementing as a separate PullSubscriber that has its own config makes sense to avoid any weirdness.