migration: Add schema formatter

Created by: efritz

Move psql formatting from schemadoc into migration package. We can re-use this shared infra for better diffing of schemas to detect drift between expected schemas and the reality of on-prem database schemas.

This nerfs schemadoc fairly hard, which I plan to roll into sg soon.

Test plan

No diff from go generate locally. And, for safety, a main-dry-run to ensure I didn't destroy CI on the main branch.