Improve return field selection and serialization

Right now in order to deserialize an embedding field, you need to call:

```
qry.return_field("embedding", decode_field=False)
qry.return_fields("title","year")
```

which is less ideally from a UX perspective. This feature was added as a simple wrapper around the `redis-py` classes. But we can try to simplify this ux for sure.