differentiate between app and user access tokens

[LosFarmosCTL]

Some API endpoints do not necessarily require a user access token, but can work with just an app access token as well. Right now, the credentials required by the TwitchClient contain a mandatory user ID, which would not be applicable for app access tokens.

There should be a differentiation between endpoints that require user access tokens and ones that do not (should be possible using generics?) and an option to create a TwitchClient with an app access token, that accepts a limited number of endpoints.