changefeedccl: pick a random coordinator node when starting changefeeds

From the docs:

> To maintain a high number of changefeeds in your cluster [..] Connect to different nodes to create each changefeed. The node on which you start the changefeed will become the coordinator node for the changefeed job.

This is surprising behaviour and not great UX. We should add an option (or better yet, make it default) to assign a random node to be the coordinator for the feed. If locality is an issue we could restrict the pool of random nodes to ones that make sense, or return an error.

Jira issue: CRDB-44658