[sql_server] Implementent purification for SQL Server Source

[ParkMyCar]

This PR implements "purification" in the Adapter for a SQL Server source. For folks unfamiliar, purification is a step during object creation where we query external systems for any necessary information, with the goal of making our persisted SQL "pure".

During purification of a SQL Server source we do the following:

1. Ensure CDC is enabled for the current database
2. Ensure snapshot isolation is enabled for the current database
3. List all tables that currently have CDC enabled, and their capture instances
   • The specified capture instance for each subsource is then persisted in `PurifiedExportDetails

Left as a TODO is implementing support for CREATE TABLE ... FROM SQL SERVER. Nothing blocks our implementation of this feature although because it's not released yet I opted out of implementing it to keep the PR small.

Motivation

Progress towards https://github.com/MaterializeInc/database-issues/issues/8762

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.

[martykulma]

minor notes/questions, nothing blocking 🚢🚢🚢

[martykulma]

Is is_cdc_enabled implicitly true here because the table has a capture instance associated with it?

There's a ensure_table_cdc_enabled above, but it doesn't look like anything calls it. Wasn't sure if it's needed by an upcoming change or if it's vestigial because we can tell that cdc is enabled for a table via this query.

[ParkMyCar]

Is is_cdc_enabled implicitly true here because the table has a capture instance associated with it?

It was! But this field also was unused so I got rid of it. Same with the ensure_table_cdc_enabled query, in a previous iteration it was used but no longer is.

[ParkMyCar]

@martykulma TFTR!