Per-room/space presence

Our presence API only gives the ability to declare presence on a global basis. From a privacy point of view, this is not ideal: what if i just want to tell users in a given room that i am currently online/offline (or a given space) without telling the world?

For instance, I might want to tell the users in a given room that my client is currently alive, if I've also stated that I'm currently participating in a video conference in that room (for matrix-native confs). I might also want to support richer presence info (eg which conference I am in from which device) - although this could be done by setting room state on a profile-as-room (but it's a bit clunky given it would need to be synced with presence online/offline state to be meaningful).

Start by reviewing the existing presence API and the room/space state model described in this issue. Define how scoped online/offline presence and richer conference or device information should work, including privacy boundaries; the issue is done when the protocol behavior and required specification changes are agreed.

Written by the indexing model from the issue text.