Model instances that contain `enum` values are not serializable

When returning an object from a FastAPI endpoint using a `response_model` that contains an enum, you current will see an error like:

```
PydanticSerializationUnexpectedValue(Expected `enum` - serialized value may not be as expected [field_name='meep', input_value=, input_type=DerivedEnumValue])
```

We should allow deserializing, and serializing enums back and forth from `str`.