fix: Try to cache DM channels during REST.

Created by: Neuheit

Summary

This PR attempts to fix DM channel caching by trying to cache it from rest, since a channel create event is no longer sent for DMs. This also allows us to fetch a cached DM channel rather than having to make a request every time, and we can ensure the state of the cache since it will be removed upon a DM channel delete.

I also made guildId nullable on the DiscordChannel object since the DiscordDMChannel inherits from it.