changefeedccl: add saved progress validator to roachtests

We can build confidence that we don't lose data due to incorrect span-level checkpoints by adding a validator that verifies the following things:

- [ ] the resolved timestamp + span-level checkpoint never regresses
- [ ] similar to the existing order validator's validation that we don't see a row for the first time after a resolved timestamp, we can do something similar with span-level checkpoints (although since they're not sent to the sink, it's a little bit more difficult)

Jira issue: CRDB-49474