Introduce an interface for a JWT library

[oxisto]

While I almost excessively use https://github.com/golang-jwt/jwt, this might be biased since I am one of the maintainers of this library. Other people might want to use other JWT libraries, so instead of having a hard dependency on jwt-go, I could image an interface for dealing with JWTs (parse from string; sign with key to string).

This would also mean that in this project I would only need to manage (direct) dependencies to the x packages of the stdlib.