Add wrap logic directly to the property methods for callables for long running operations

[ohmayr]

Instead of implementing _prep_wrapped_messages in the base class and overriding it in the async classes, we should wrap the logic directly within the property callable methods and get rid of this method (both from the base class and the subclasses).

What we have now:

@property
    def get_operation(
        self,
    ) -> Callable[
        [operations_pb2.GetOperationRequest],
        Coroutine[Any, Any, operations_pb2.Operation],
    ]:
        return self._get_operation

Instead we could do something like:

    @property
    def get_operation(
        self,
    ) -> Callable[
        [operations_pb2.GetOperationRequest],
        Coroutine[Any, Any, operations_pb2.Operation],
    ]:
        return gapic_v1.method_async.wrap_method(
                self.list_operations,
                default_retry=retries_async.AsyncRetry(
                    initial=0.5,
                    maximum=10.0,
                    multiplier=2.0,
                    predicate=retries_async.if_exception_type(
                        core_exceptions.ServiceUnavailable,
                    ),
                    deadline=10.0,
                ),
                default_timeout=10.0,
                client_info=self.client_info,
            ),

[vchudnov-g]

An memoizing the wrap_method call, so we don't call it again each time. We did something similar in a recent PR to gapic-generator-python, though I'm not finding the code just yet.