Default sort

It would be great if `SQLAlchemyObjectType` allowed to configure a `default_value` of a `#sort_argument` method result.

```python
def sort_argument_for_object_type(
# ...
return Argument(List(enum), default_value=enum.default)
```

Right now there is no way to configure the `default_value` of the `Argument`.

Start at SQLAlchemyObjectType and locate the sort_argument method described in the issue, then inspect how its Argument is constructed. The change is complete when callers can configure the Argument's default_value for the sort result; no test location is provided in the issue.

Escrita pelo modelo de indexação a partir do texto da issue.