[persist] Allow disabling the sink progress collection optimization

[bkirwi]

Motivation

Discussed live: easy and possibly useful.

Tips for reviewer

Note that this doesn't allow disabling the creation etc. of the new shard or the writes in the Kafka sink itself, since that involves wider changes to the controller that are hard to separate out. Still, this may be useful in the case where that stuff is fine but the actual optimization itself is unsafe...

I still have a couple TODOs I'd like to get to ~soonish, but I'll split those out to a separate PR to increase the odds this one merges by the release cut.

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.

[ParkMyCar]

LGTM, thanks for adding this flag!