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

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

df = pl.DataFrame({"a": ["cow", "monkey", "hippopotamus", None]}).lazy()
df = df.with_columns(padded=pl.col("a").str.pad_end(8, "*"))

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

NotImplementedError: String function <Name.PadEnd: 23>

Describe the solution you'd like
I'd like the above code to be able to execute using the polars GPU backend. This should map nicely to cudf::strings::pad with side_type::RIGHT.

Describe alternatives you've considered
N/A

Additional context
#16480