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

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

df = pl.DataFrame({"a": [-1, 123, 999999, None]}).lazy()
df = df.with_columns(zfill=pl.col("a").cast(pl.String).str.zfill(4))
res = df.collect(engine=engine)
print(res)

NotImplementedError: String function <Name.ZFill: 44>

Describe the solution you'd like
I'd like the above code to be able to execute using the polars GPU backend. libcudf supports cudf::strings::zfill so this may map nicely 1-1.

Describe alternatives you've considered
N/A

Additional context
#16480