Allow to only save only dataframes instead of both fnames and derived fnames

Also allow to pass these kwargs:
```
def save_dataframe(
    fname: str | list[str],
    *,
    format: _DATAFRAME_FORMATS = "parquet",  # noqa: A002
    save_kwargs: dict[str, Any] | None = None,
    expand_dicts: bool = True,
    **to_dataframe_kwargs: Any,
) -> Callable[[adaptive.BaseLearner], None]:
```