Extra returning fields in `UpdateRequest.apply`

[kigawas]

Sometimes there is need to retrieve some server updated fields in the in-memory instance:

user = await User.create()
print(user.updated_at) # updated_at_1
await user.update(balance=User.balance + 100).apply()
print(user.updated_at) # updated_at_2, and it should not be equal to updated_at_1

Unfortunately, gino only retrieves updated fields like

 # in `crud.py`
 clause = (
 type(self._instance)
 .update.where(
 self._locator,
 )
 .values(
 **self._instance._get_sa_values(values),
 )
 .returning(
 *[getattr(cls, key) for key in values], # <- only updated values will return
 )
 .execution_options(**opts)
 )

I'd like to specify some "extra fields" here, maybe in this form:

user = await User.create()
print(user.updated_at) # updated_at_1
await user.update(balance=User.balance + 100).apply(extra_returning_fields=['updated_at'])
print(user.updated_at) # expected: updated_at_2 > updated_at_1