RFC 311: Expose migration status via GraphQL

Created by: efritz

Part of RFC 311: Out of band migrations.

Expose a GraphQL API to modify the data in the out-of-band migrations table introduced in https://github.com/sourcegraph/sourcegraph/issues/18237. The major operations here should include:

• read current progress of each listed migration
• toggle the direction of a particular migration (for downgrading)