cdc: handle pts contention better

When running many changefeeds separately, the protected timestamp system can get overwhelmed, and we don't seem to have many knobs to tune how often we make calls. We should do some or all of the following:

- [ ] Add/document a cluster setting to reduce the number of calls a changefeed makes to advance protected timestamps.
- [ ] Don't restart the feed when we fail to advance a protected timestamp
- [ ] Retry with backoff when trying to create a protected timestamp
- [ ] Automatically throttle the rate at which we advance protected timestamps in response to backpressure from the pts.

Jira issue: CRDB-25344

Epic CRDB-28882

Start by reviewing the changefeed and protected timestamp concerns described in this issue, along with Jira CRDB-25344 and epic CRDB-28882. The issue lists four possible directions, so the scope and completion criteria need to be clarified before implementation.

Written by the indexing model from the issue text.