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

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

df = pl.DataFrame(
    {"json": ['{"a":1, "b": true}', None, '{"a":2, "b": false}']}
).lazy()
dtype = pl.Struct([pl.Field("a", pl.Int64), pl.Field("b", pl.Boolean)])
df = df.with_columns(decoded=pl.col("json").str.json_decode(dtype))


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

 NotImplementedError: dtype=Struct({'a': Int64, 'b': Boolean}) 

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 struct dtypes, and lack of a dedicated libcudf API, and while I don't see one that exists today, a libcudf API that decodes json into a struct dtype is something I can see existing at some point.

Describe alternatives you've considered
N/A

Additional context
#16480