Switch rooms directly without explicitly leaving old and joining new.

[fancycode]

One possible optimization:

• [ ] When switching rooms, clients should not leave the room before joining the new one. Currently clients send a room request with an empty room id (which leaves the current room) and then another room request with the new federated room. Switching directly would allow to reuse the federated connection.

Originally posted by @fancycode in https://github.com/nextcloud/spreed/issues/12604#issuecomment-2236749121