Create an interface to abstract the upstream IDP from the
authorize, IDP discovery, callback, choose IDP, and login
endpoints. This commit does not refactor the token endpoint,
which will be refactored in a similar way in the next commit.
Made the switch wherever possible, but since fosite still uses the old
gopkg.in/square/go-jose.v2 there was one test where we still need to use
it as a direct dependency.
