[FEA] Support polars.Expr.str.head 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.head. Running the example from the docs yields:

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

df = pl.DataFrame({"s": ["pear", None, "papaya", "dragonfruit"]}).lazy()
df = df.with_columns(pl.col("s").str.head(5).alias("s_head_5"))

res = df.collect(engine=engine)
print(res)

NotImplementedError: String function <Name.Head: 14>

Describe the solution you'd like
I'd like the above code to be able to execute using the polars GPU backend. This API returns the first n characters of all the strings, accepting both a scalar argument or a column argument containing many n for an elementwise version. At first glance it seems these may map nicely to each overload of cudf::strings::slice_strings.

Describe alternatives you've considered
N/A

Additional context
#16480