oobmigrations: Expose `conf` to migrators

Created by: efritz

This PR does an (almost final) refactor to isolate oobmigration runners from direct calls to conf. All DSN and site configuration values are passed as an explicit parameter to the (new and currently unused) migration registration function. Fixes #40392 (closed).

After #40395 is validated and merged we will be able to run any oobmigration in the migrator without a running instance.

Test plan

Existing CI pipelines.