enh: add LazyFrame.pivot

[MarcoGorelli]

This is something I'd quite like to see. In Polars too, but it's easier to implement here 😄

There's an example in the Polars docs about how to do a lazy pivot: https://docs.pola.rs/api/python/stable/reference/dataframe/api/polars.DataFrame.pivot.html

API-wise, we could have:

    def pivot(
        self: Self,
        on: str | list[str],
        *,
        column_names: list[str],
        index: str | list[str] | None = None,
        values: str | list[str] | None = None,
        aggregate_function: Literal[
            "min", "max", "sum", "mean", "median", "len"
        ]
        | None = None,
        separator: str = "_",
    ) -> Self:

This is very similar to DataFrame.pivot, but requires specifying the output column names explicitly so that the computation can stay completely lazy