Expose presence on ARTRealtimeChannelProtocol

[umair-ably]

Presence is needed to implement presence and typing indicators. I'd expect to get the realtime channel presence instance via realtime.channels.get(...).presence, however this is not exposed publicly.

It seems like presence lives on ARTRealtimeChannel (which in turn has an ARTRealtimePresenceInternal implementation) and not on ARTRealtimeChannelProtocol.

Expected behaviour:
Be able to call channel.presence from an ably-cocoa implementing SDK e.g. chat

┆Issue is synchronized with this Jira Task by Unito