changefeedccl: start watching new targets at ALTER CHANGEFEED statement time

Currently, new targets added with ALTER CHANGEFEED are watched as of the changefeed's current highwater. It would make more sense to instead start watching the new targets as of the ALTER CHANGEFEED statement time. This isn't currently easy to implement because the changefeed infrastructure doesn't have a way of doing initial scans for different tables at different timestamps. It would probably also be nicer if we didn't start sending events for the new targets until the highwater reaches the ALTER statement time, which is another thing that isn't currently supported.

Relates to #83181
Relates to #142376
See also https://cockroachlabs.slack.com/archives/C065X5307U3/p1772029389997969

Jira issue: CRDB-60860