Polars DataFrame Support

[Polars](https://github.com/pola-rs/polars) is a single-machine columnar dataframe library built on top of pyarrow Tables with both a eager and lazy API. While the default pickle serialization works fine for eagerly-created dataframes, performance could be improved by leveraging the same IPC serialization pyarrow Tables currently have. Additionally, LazyFrames are not currently serializable, but contain an underlying json plan that could be passed over the wire.