add video + audio channels possibility to a rtc p2p connection

In the matrix signaling scenario, and the matrix.actor endpoint.

matrix.actor/@a:matrix.org#call_from=@b:matrix.org a link to create a call from b to a if user a is on the matrix endpoint: matrix.actor#call_from=* matrix.actor/@a:matrix.org#call_from=@b:matrix.org a will receive the call from b