Pickup provider name from go 1.22 mux

[pinpox]

Using the new mux addet to go 1.22 natively, the provider name is not picked up. It mostly replaces gorilla/pat in many places, e.g. path parameters.

There is the alternative of working around this like this:

	mux.HandleFunc("GET /auth/{provider}", func(res http.ResponseWriter, req *http.Request) {
		// try to get the user without re-authenticating

		// goth doesn't pick up the provider using the go 1.22 new routers, so
		// we work around by setting it in the context.
		req = req.WithContext(context.WithValue(req.Context(), "provider", req.PathValue("provider")))

		if gothUser, err := gothic.CompleteUserAuth(res, req); err == nil {
			err = templates.ExecuteTemplate(res, "user.html", gothUser)
			if err != nil {
				panic(err)
			}
		} else {

			gothic.BeginAuthHandler(res, req)
		}
	})

By merging this PR it would no longer be necessary to set the provider explicitely in the context of the request.