fix(server): do not get client by id for introspection

As introspection is a Oauth mechanism for resource servers only,
it does not make sense to get an oidc client by ID.
The original OP did not do this and now we make the server behavior similar.

pkg/op/token_intospection.go

@@ -65,3 +65,8 @@ func ParseTokenIntrospectionRequest(r *http.Request, introspector Introspector)
 
 	return req.Token, clientID, nil
 }
+
+type IntrospectionRequest struct {
+	*ClientCredentials
+	*oidc.IntrospectionRequest
+}