migration: Add Describe method to store

Created by: efritz

Pulled from #32472. This PR adds a Describe method to the migration store that queries system catalogs to describe the current schema (with the same level of information as \d+ that powers our schemadoc tool).

Eventually this data can be used to describe the current schema and detect drift from an expected golden standard. This can help us detect things like malformed tables, missing indexes, etc, that have occurred during a failed upgrade by hand.

Test plan

New unit tests.