RFC 311: Write basic migration machinery

Created by: efritz

Fixes https://github.com/sourcegraph/sourcegraph/issues/18353. This PR adds oobmigration.Runner, which is a registry and orchestrator of out-of-band migrations. Teams can register migrations to be run against a specific migration record in the database (with a known, unique identifier), and the runner will take care of continually calling Up/Down on the migrator until it has completed.

Follow-up issues:

• https://github.com/sourcegraph/sourcegraph/issues/18406
• https://github.com/sourcegraph/sourcegraph/issues/18407
• https://github.com/sourcegraph/sourcegraph/issues/18408