refactor!(realtime_client): some realtime refactor

[Vinzent03]

Breaking changes

• move types from realtime_channel.dart to types.dart
• make the error parameter for .subscribe() non optional
• make the status parameter for subscribe() an enum instead of a String
• add some documentation to ChannelFilter, because I found no way to use an enum for ChannelFilter.event, because for broadcast it can be any string

I think that's it for now that can be type wise improved.

[dshukertjr]

Looks beautiful!