Separating raw queries from query builder

Right now we support both at the same time:

```python
# Raw query
await Movie.query({"name": "X"}).first()

# Query builder
await Movie.query(Movie.name == "X").first()
```

This leas to complexity handling both in the same method. I'm thinking maybe adding a `.raw()` method which accepts the query and doesn't do any validation/query building and sends it directly to MongoDB.