[FEA] Support `polars.Expr.str.to_decimal` 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. to_decimal](https://docs.pola.rs/api/python/stable/reference/expressions/api/polars.Expr.str.to_decimal.html). Running the example from the docs yields:
```
import polars as pl
engine = pl.GPUEngine(raise_on_fail=True)

df = pl.DataFrame(
{
"numbers": [
"40.12",
"3420.13",
"120134.19",
"3212.98",
"12.90",
"143.09",
"143.9",
]
}
).lazy()
df = df.with_columns(numbers_decimal=pl.col("numbers").str.to_decimal())
res = df.collect(engine=engine)
print(res)
```

```
NotImplementedError: dtype=Decimal(precision=None, scale=None)
```

**Describe the solution you'd like**
I'd like the above code to be able to execute using the polars GPU backend. This is blocked on decimal dtypes.

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

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