Add extras for presence messages

[jamienewcomb]

tiholic to add spec item

┆Issue is synchronized with this Jira Task by Unito

[tiholic]

Requirement is to support extras on realtime.channel.presence.enter, realtime.channel.presence.update, realtime.channel.presence.leave, realtime.channel.presence.enterClient, realtime.channel.presence.updateClient and realtime.channel.presence.leaveClient

Related spec items:

https://docs.ably.io/client-lib-development-guide/features/#TP3i
https://docs.ably.io/client-lib-development-guide/features/#RTP8e
https://docs.ably.io/client-lib-development-guide/features/#RTP9a
https://docs.ably.io/client-lib-development-guide/features/#RTP10a

As no spec exists on how to what to test for PresenceMessage#extras, referring to testing published messages with extras: https://docs.ably.io/client-lib-development-guide/features/#RSL6a2

[ikurek]

I've checked both ably-cocoa and ably-java and I couldn't find the extras handled anywhere, so I guess this would need to be implemented on native libraries first