changefeedccl: detection of descriptor changes

[danhhz]

Changefeeds currently work by turning a user-provided list of tables into a set of kv spans to watch and setting up watches on them that emit changed data into a buffer (currently polling via ExportRequest and soon push via RangeFeed). Separately, the buffer is continually read and the changes in it are turned into rows and emitted.

One thing that hasn't yet been solved satisfactorily is how to detect when the set of watched spans change (so that we can handle it). Some situations when this could happen:

1. Once we support watching a database and a table is added to the database
2. Once we support watching a partition and the partition definition changes
3. Once we support change changing the primary index on a table

A closely related issue is detecting schema changes:

4. We decided in the rfc that attempting to truncate a table being watched by a changefeed will reject the truncate with an error (there's no performant and unsurprising way for us to deal with this in the changefeed) but it's not clear how this will work in the core version (which doesn't have a jobs entry marking the table as watched). Currently, the behavior of both enterprise and core is that truncates are detected and error the changefeed.
5. The semantics of schema changes that require a backfill are tricky: changefeedccl: support (or workaround) schema changes with backfills #28635

All of these are derivable from table and database descriptor changes.

I'm going to use this issue as a log of my thoughts on how to solve this problem.

[danhhz]

Turns out this was more straightforward than I thought. #29367 does the hard bits and everything left is naturally part of #28633, so closing this