feature: Add SocketUnknownChannel

[SubZero0]

This is another way of resolving, partially, the issue with Threads.
It has a lot less changes than the other ( #1901 ), but with the disavantage that nothing will apparently break for who's using Discord.Net.

This solution adds a new channel type, SocketUnknownChannel, that inherits from a ISocketMessageChannel so making it easy to do basic functionality (send messages, files, get messages, etc), but has the drawback that now the Name property could be null, as the only information received about it is the id.

So while it would fix the issue of not being able to process messages without a cached channel, it might cause a few unexpected issues for the developer, also by adding the possibility of a third type of "basic" channel (private, guild, unknown).

I still need to check the structure for the parsers and attributes to see if they are still working as expected.

Possible changes

• Rename SocketUnknownChannel to SocketUnknownMessageChannel

Changes

• Add a new channel type: SocketUnknownChannel
  This channel might be present inside message objects returned by the events MessageReceived and MessageUpdated.

[quinchs]

The idea for an unknown entity type is good for when discord initially releases new entity types, people running older version don't have to update to fix breaking changes. I think we should have different types of unknown channels ex: SocketUnknownVoiceChannel, SocketUnknownTextChannel, etc or have a generalized SocketUnknownChannel with just the id.