Data Sharing: Endpoints to get active subscriptions and backfill requests

[oneiros]

See #36 (comment)

If a FASP experiences data loss it might be good to be able to get the current state of subscriptions and backfill requests from the server.

This is not strictly necessary to get started, so I have not prioritized this for now, but for a more complete solution we can confidently call 1.0 we should definitely consider adding this.