[FEA] Support polars.Expr.str.split in cudf-polars

[brandon-b-miller]

Is your feature request related to a problem? Please describe.
Today on branch-25.08, we lack support for polars.Expr.str.split. Running the example from the docs yields:

import polars as pl
engine = pl.GPUEngine(raise_on_fail=True)

df = pl.DataFrame({"s": ["foo bar", "foo_bar", "foo_bar_baz"]}).lazy()
df = df.with_columns(
    pl.col("s").str.split(by="_").alias("split"),
    pl.col("s").str.split(by="_", inclusive=True).alias("split_inclusive"),
)
res = df.collect(engine=engine)
print(res)

NotImplementedError('String function <Name.Split: 29>')

Describe the solution you'd like
I'd like the above code to be able to execute using the polars GPU backend. There's some string splitting functionality in libcudf but in most cases it returns a table with the split elements. In addition, since this API returns a list[str] dtype, it's blocked on list dtypes generally in cudf-polars.

Describe alternatives you've considered
N/A

Additional context
#16480