Allow sending signaling messages to "call" in addition to "room"

[SystemKeeper]

• Ref: Send signaling message to room instead of to each participant when sending to all of them nextcloud/spreed#12881

Currently we can send signaling messages to "room" and the signaling server will take care of sending it to each participant currently in the room. It would be nice to also have the possibility to send to "call", so all messages would be send to participants currently in the call.