Stream and Consumer leader down placement hints

[ripienaar]

Proposed change

Like io.nats.jetstream.api.v1.meta_leader_stepdown_request lets support placement hints for stream and consumer step down, hinting and who should be the next leader.

Use case

nats stream cluster balance attempts to balance all the stream leaderships in the cluster, does so by only picking the fewest streams to move and then initiate leader elections on them.

But since the outcome is random it's quite hard to make this work well, we'd like a hint to suggest the next leader node - if impossible normal placement to continue.

Contribution

No response