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

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:

```python3
@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:
```python3
@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,
),
```