changefeedccl: add to db console information about throttling

As part of the work to make changefeeds handle quotas more gracefully (plus the work we did for admission control), changefeeds can quietly end up in a throttled state for many reasons. We should indicate when changefeeds are in a throttled state in the db console:
- We track how much time we spend paused due to throttling because of kafka quotas, we should surface this in a graph
- We also know per node/ per processor what byte rate are we targeting, we should surface that in a graph

See https://github.com/cockroachdb/cockroach/pull/104446
Jira issue: CRDB-28927