adapter: read-only mode

[aljoscha]

Work towards https://github.com/MaterializeInc/materialize/issues/27406

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).
• [ ] This PR includes the following user-facing behavior changes:

[def-]

I added some tests, but maybe this was too easy? I expected more problems: https://github.com/MaterializeInc/materialize/pull/27841/commits/de78d2401de77f426d3eb70a81f72bc7297fab67

[aljoscha]

I added some tests, but maybe this was too easy? I expected more problems: de78d24

I didn't expect problems 😅

[aljoscha]

Superseded by all the PRs that have been spun out from this