add gorilla mux adapter

[moqsien]

No description provided.

[its-felix]

Hey, thank you for the contribution!

Since *mux.Router implements http.Handler, there is no need for a separate gorilla mux adapter. You can simply use the vanilla adapter with gorilla mux:

func main() {
	r := mux.NewRouter()
	r.Handle("/ping", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request){
		w.WriteHeader(http.StatusOK)
		w.Write([]byte("pong"))
	}))

	adapter := adapter.NewVanillaAdapter(r)
}