Added copy constructor and assignment operator to UserView.
Added copy constructor and assignment operator to UserView.

PiperOrigin-RevId: 264679967

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk