Missing support for jetstream consumer groups

[oliverlambson]

Proposed change

Server 2.11 introduced jetstream consumer priority groups from ADR-42 (2.11 release notes for initial addition, 2.12 release notes for prioritized policy addition).

From looking at the original nats.go implementation in nats-io/nats.go@93f7ce4c, I believe the translation of this would roughly be:

• 3 new js consumer config fields: nats.js.api.ConsumerConfig.{priority_policy,priority_timeout,priority_groups}
• 3 new options on nats.js.client.JetStreamContext.{pull_subscribe(),pull_subscribe_bind()} (or could just rely on sdk user manually creating multiple consumer configs for the same consumer group to configure these themselves): pull_min_pending, pull_min_ack_pending, pull_priority_group
• modification to the nats.js.client.JetstreamContext.PullSubscription.{_fetch_one(),_fetch_n()} methods to handle priority groups in the pull request: need to add id, group, min_pending, min_ack_pending, priority
• 1 new js api method: nats.js.manager.JetstreamManager.unpin_consumer()
• 1 new js error: nats.js.errors.PinIdMismatchError (error code 423)
• 1 new js consumer info field: nats.js.api.ConsumerInfo.priority_groups

Use case

Would then allow the implementation of, among other things, partitioned consumer groups – which are super useful for doing things like event sourcing without having to set max ack pending to 1 and slowing a consumer to a crawl

Contribution

I'm happy to make a PR I have made a PR: #786