kvfollowerreadsccl: plug bounded staleness into the execution engine

Jesse Seldess commented:

PR: https://github.com/cockroachdb/cockroach/pull/68967

From release notes:
> Errors involving `MinTimestampBoundUnsatisfiableError` during a bounded staleness read now get a custom `pgcode` (`54C01`). [#68967][#68967]

Jira Issue: DOC-1381