Allow ProofOfPossesion Options per Endpoint

[nenaraab]

For a feature version (like 1.0) we can think of solution how to integrate this check into the middleware.

In our project we check all authentication with the help of http subrouters and middlewares before the actual endpoint.

subrouter.Use(
    middleware.AuthenticationHandler
)

A way to customise this Middleware and enable/disable parts of the validation based on the route would be really cool. Maybe something like the following:

func (m *Middleware) CreateAuthenticationHandler(proofOfPossession options) func(handler http.Handler) http.Handler {
    // return the actual AuthenticationHandler with options enabled/disabled
    // and checks like azp == expected caller client id
}

Originally posted by @f-blass in #35 (comment)