Signalling message: Add to group

Adds a channel to a group. The channel name is the server channel name. The group may be set to expire after a certain amount of time (ttl) in seconds. The expiry time is reset each time a channel is added to the group. If channel is already part of the group then adding it again will not add the channel a second time, but will reset the expiry time.

This message blocks the client thread until a response is received from the server.

Action	Client -> Server	Server -> Client
Initial action	Message: b"GROUPADD" Properties: group_name (bytes): Name of group. channel_name (bytes): Name of channel. ttl (float): Time until group expires.	Add channel to group. Respond: -> Complete -> Exception
Complete	Message: b"COMPLETE"	No further action.
Exception	Message: b"EXCEPTON" Properties: exception (str): Exception description.	No further action.