changefeedccl: show changefeed jobs delegate query has hard to parse CTEs

**Is your feature request related to a problem? Please describe.**
With DB-level changefeeds, and especially withtheir need to apply table filters, the delegate query for SHOW CHANGEFEED JOBS has many CTEs and has become hard to reason about.

**Describe the solution you'd like**
SHOW CHANGEFEED JOBS should be implemented as an opaque statement.

**Describe alternatives you've considered**
I implemented the query as a delegate [https://github.com/cockroachdb/cockroach/pull/156873](url), but it's difficult for reviewers to follow. I've attempted a few different ways, but the intermediate CTEs remain difficult to parse.

May relate to https://github.com/cockroachdb/cockroach/issues/146640

Jira issue: CRDB-56942

Epic CRDB-55919