[dart_sdk] Gateway support

The gateway is finished, there are some ongoing work related to data parsing and data mapping but thats a separate issue.

@M0n7y5 Do dm and channel messages update in real time now or is that not apart of this ticket?

@edeuss dms do partially update but channel are not, tho that should be and issue related to dms or channel, this stuff needs to be handled in the app, the sdk has everything you need to fix those issues tho