storage/sources: Planning for CREATE TABLE .. FROM SOURCE statements

[rjobanp]

Motivation

• This PR adds a known-desirable feature. Fixes https://github.com/MaterializeInc/materialize/issues/28953 and Fixes https://github.com/MaterializeInc/materialize/issues/28952

Tips for reviewer

Each commit is a semantic change, and at least one or two are just refactoring, so it will be easier to grok when reviewed commit-by-commit.

I've added basic tests of the functionality to testdrive, but will require assistance from the QA team to port over many of the extensive tests on subsources to use this new format while we test internally.

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.

[shepherdlybot[bot]]

Mitigations

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• [ ] (Required) Run Nightly Tests
• [ ] Unit Test

Risk Summary:

The pull request has a high-risk score of 80, primarily driven by predictors such as "Sum Bug Reports Of Files" and "Delta of Executable Lines," with 6 modified files identified as hotspots. Historically, PRs with these predictors are 108% more likely to cause a bug than the repository baseline. While the observed bug trend in the repository is decreasing, the predicted bug trend is on the rise.

Note: The risk score is not based on semantic analysis but on historical predictors of bug occurrence in the repository. The attributes above were deemed the strongest predictors based on that history. Predictors and the score may change as the PR evolves in code, time, and review activity.

Bug Hotspots: What's This?

File	Percentile
../catalog/migrate.rs	96
../src/sources.rs	98
../src/coord.rs	100
../sequencer/inner.rs	97
../catalog/open.rs	96
../src/lib.rs	99