adapter: actually treat CREATE TABLE ... FROM WEBHOOK as a Table

[ParkMyCar]

Some refactoring in the Adapter to treat objects created by CREATE TABLE ... FROM WEBHOOK actually as tables. This way they show up when querying mz_tables, you can run SHOW CREATE TABLE ..., etc.

Motivation

Fixes https://github.com/MaterializeInc/database-issues/issues/8862

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.

[jkosh44]

LGTM, do we need to update the migration PR to migrate all source webhooks to tables?

[ParkMyCar]

LGTM, do we need to update the migration PR to migrate all source webhooks to tables?

Yeah we should, I'll file an issue for that