feat: Multi-participant calls

[xipasduarte]

Ad multi-participant call support. Maybe set a some limit, but try to add 3 to 5 in an early approach, if there are any technical issues.

Motivation
Although the main feature is pairing, and we only do that 1-on-1, there is a need for external calls on some issues related to the work, that another dev might help with.
Currently we need to break from the pair and enter a room, to be able to to this, which breaks the flow. Also, rooms aren't private, and we open ourselves to be interrupted by others, without fault of their own.

[iparaskev]

This makes sense. We are currently working on #258 (webrtc migration to rust). Once we are done with this major refactoring we can start working on this. It should be quite straightforward to support it.