Native Polars DataFrame support

### Feature Request

Currently `group.as_dataframe()` will return a pandas DataFrame.
It would be great to support [Polars](https://github.com/pola-rs/polars) DataFrame for better performance. Thanks! ☺️

### Workaround Solution

```py
import polars as pl

# ...
df = pl.from_pandas(group.as_dataframe())
```