Narwhalify `from_group_dataframe()`

[dennisbader]

• Since we refactored from_dataframe() in Narwhals implementation of from_dataframe and performance benchmark #2661 leveraging narwhals to support more DF backends, we should also refactor from_group_dataframe().
• Also, investigate whether we can speed up things as explained in Speed up parallel from_group_dataframe #2645.