[FEA] Support `polars.Expr.str.decode` in `cudf-polars`

**Is your feature request related to a problem? Please describe.**
Today on `branch-25.08`, we lack support for [polars.Expr.str.decode](https://docs.pola.rs/api/python/stable/reference/expressions/api/polars.Expr.str.decode.html). Running the example from the docs yields
```
import polars as pl
engine = pl.GPUEngine(raise_on_fail=True)

df = pl.DataFrame({"color": ["000000", "ffff00", "0000ff"]}).lazy()
df = df.with_columns(pl.col("color").str.decode("hex").alias("decoded"))

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

```
NotImplementedError: dtype=Binary conversion not supported
```

**Describe the solution you'd like**
I'd like the above code to be able to execute using the polars GPU backend. There's not a native `libcudf` API I can find that maps to this operation directly.

**Describe alternatives you've considered**
N/A

**Additional context**
https://github.com/rapidsai/cudf/issues/16480