migrator: Add skeleton service
Created by: efritz
Add no-op migrator service. Partial effort towards https://github.com/sourcegraph/sourcegraph/issues/25253. Next steps will be to include this in CI pipelines (https://github.com/sourcegraph/sourcegraph/issues/25255 and https://github.com/sourcegraph/sourcegraph/issues/25256), and expanding this service to actually run migrations synchronously.
Once this is in place we can stop running migrations on application startup and replace it with a check to ensure it's already up to date.