add DiscordGuild.GetChannelAsync

Created by: ExaInsanity

The channel could already exist, but does not yet need to be cached - which may be an uncommon race condition with GetChannel, but it is not too unrealistic. In fact, it can be observed a fair amount with high-latency connections, either when the user creates a channel that is instantly used by the application, or when the application creates a channel on one thread and requests it on another.

Notes

This may cause performance regressions in the event an application keeps requesting non-existent channels. To my knowledge, there is no way to know whether a channel exists before requesting it from the API, though.