deletes DSharpPlus.Rest and instead moves its feature set into DSharpPlus

this is a first step towards properly enabling OAuth2 usage, and more clean-up, documentation and polishing will be needed. it also reduces the maintenance effort required for raw rest access, since we no longer have to keep DSharpPlus.Rest up to date (which it wasn't).

the default logger ignores HttpClient logging spam entirely, and users using 3rd party loggers will have to set log level overrides.