[WIP] Support ordered first/last groupby aggregations in cudf-polars - #23600

Closes #23592

Adds cudf-polars support for grouped sort_by(...).first() and sort_by(...).last() aggregations, including streaming decomposition for multi-partition execution.

This enables expressions like:

df.group_by("g").agg(
    pl.col("x").sort_by("t").first().alias("open"),
    pl.col("x").sort_by("t").last().alias("close"),
)