ModelSelect public method to return list of columns in the SELECT
Added `selected_columns` as a property.
Unless I am mistaken, there is no public way to get the list of columns in the select.

The only way I see is to call the private `._returning` attribute.

Can we please add public access for this? SQLAlchemy has `selected_columns` in 1.3/1.4. In previous versions you could use the `.c` which returned a list of selected columns.

```
query = Tweet.select(
    Tweet.id,
    Tweet.content,
    User.username
).join(User)

selected_columns = query._returning

print(selected_columns)
```