Manual promotion support

[alex-hunt-materialize]

Manual promotion support

Motivation

• This PR adds a known-desirable feature.
  Part of https://github.com/MaterializeInc/database-issues/issues/9872

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[alex-hunt-materialize]

Waiting to write tests for merge this until #34214 is in, as this will need changes to the test procedure, and I don't want to deal with conflicts.

[alex-hunt-materialize]

I found one bug while implementing this, which we can solve separately. This code doesn't make it worse, at least.
https://github.com/MaterializeInc/database-issues/issues/9918