storage/sources: Allow user-specified column names in CREATE TABLE FROM SOURCE statements

[rjobanp]

Motivation

• This PR fixes a recognized bug. Fixes https://github.com/MaterializeInc/database-issues/issues/8597

The issue above highlighted that the new CREATE TABLE .. FROM SOURCE statements did not allow the specification of column-names that we currently allow in CREATE SOURCE statements for some source types (Kafka sources and single-output load-generator sources).

This PR refactors our SQL parsing and purification/planning logic to accommodate this behavior.

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.

[jkosh44]

LGTM