First draft at returning strawberry types from mapped resolvers.

[mattalbr]

Strawberry doesn't actually type check the values returned from resolvers -- the "self" in resolvers is really just whatever object the parent resolver returned, transparently forwarded by strawberry. Before this change, all of our resolvers return (and in turn only support) "self" being an instance of a sqlalchemy model, not the strawberry type.

At best, that's confusing, but at worst, it doesn't allow manually defined resolvers on a mapped type to call sibiling resolvers. E.g. imagine a user-defined resolver:

```python
@mapper.type(models.MyModel)
class MyModel:
  def resolve_other_field(self, info) -> int:
    return 5

  def resolve_some_field(self, info) -> int:
    return self.resolve_other_field(info) + 5
```

This fails, because the sqlalchemy model type has no "resolve_other_field" defined, which is perplexing, because it seems like it should be defined. The only way to call it is like: MyModel.resolve_other_field(self, info), but IMO this unnecessarily exposes the user to strawberry internal knowledge (static method nonsense) that they don't need to be privvy to. By instead ensuring all of our resolvers return the strawberry type, all the typing works just like expected.