Investigate: Expose API(s) to stop ongoing background migrations and provide status of background migrations

Following conversation in: #1374 (comment 2228235359)

We should provide a way to (at least) stop,start,paus,resume ongoing background migrations and provide status of background migrations (remotely) without requiring direct interaction with the registry binary/deployment. An ideal way to do this would be via an API. That's also how it's done for Rails, and it's integrated with chatops:

https://docs.gitlab.com/ee/development/database/batched_background_migrations.html#managing. Also consider how this API will be accessed (e.g. chat-ops integration for the API)