Feature/graceful reconfig v1 [WIP]

[jubrad]

Work in progress for graceful reconfigugration

• DDL parsing and planning
• integration with the sequencer
• feature flag

Motivation

https://github.com/MaterializeInc/materialize/issues/20010

Tips for reviewer

Right now, this is a rough draft. I'm just trying to get things running and validated before cycling back. The pr is largely opened to discuss the direction of the implementation and get some help with some parts I'm unsure about.

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: