Make somebody a co-host of a live call, or stand them down again.

Gives somebody the call controls — muting, removing, and deciding who may speak — for the length of one call, or takes them back. Available to anybody who already has them. The chatroom owner and admins are not affected either way: they hold these controls in every call in their room, so they cannot be given them and, more importantly, cannot be stripped of them by a guest. A change takes effect the next time that person connects to the call, because what somebody is allowed to do is fixed in the joining pass they were issued; ask them to reconnect, or call join_call_session for them again. The appointment lasts as long as the call and does not make anybody an admin of the chatroom.