Make _ListWrapper implement all list methods, add append only methods to List.

By extending list, _ListWrapper is both a list, and a pointer to a list
(self._storage). As such we need to be careful to overload all methods from
list to avoid having two places where items could be read or written to.

To try and prevent this issue coming up again I've added a test which ensures
that all methods from list are overriden.

PiperOrigin-RevId: 230695450