adapter/sources: Support custom timelines on source tables to handle CdCv2 envelope type correctly

[rjobanp]

Motivation

• This PR fixes a recognized bug. Fixes https://github.com/MaterializeInc/database-issues/issues/8649

The above bug demonstrated hung queries whenever using a CREATE TABLE .. FROM SOURCE .. ENVELOPE MATERIALIZE statement, since ENVELOPE MATERIALIZE (also known as Envelope::CdcV2) uses a non-default timeline by default.

This PR introduces the ability to set a custom timeline on CREATE TABLE .. FROM SOURCE and to correct the default timeline when using ENVELOPE MATERIALIZE, matching the existing timeline handling semantics of CREATE SOURCE statements.

The tests for this are in @nrainer-materialize 's PR #29801 and I tested this change on his branch locally to verify the fix.

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.