(feature) ActivityPub Actor per collection?

In addition to the generic @relay ActivityPub actor, it would be great to have per-collection actors as well. This would enable users in the Fediverse to follow a particular collection only.

What would be necessary to do that? (I might try and create a PR).

Would this be possible at all? Because in #365, you mention that changing the actor would break the federation. With collection-based actors, we would have dynamic actors that could appear or disappear on the creation/deletion of collections.