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

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

df = pl.DataFrame({"a": ["Café", "345", "東京", None]}).lazy()
df = df.with_columns(
    pl.col("a").str.len_bytes().alias("n_bytes"),
    pl.col("a").str.len_chars().alias("n_chars"),
)

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

NotImplementedError('String function <Name.LenChars: 20>```

Describe the solution you'd like
I'd like the above code to be able to execute using the polars GPU backend. This API simply returns the length of each string in bytes, there doesn't seem to be a public libcudf API that does this today but cudf::string_view does expose the size of the string in bytes, so this should be fairly simple to add in theory.

Describe alternatives you've considered
N/A

Additional context
#16480

[davidwendt]

This seems to be the same as #19000
Do we need 2 issues for this?

[brandon-b-miller]

The two are slightly different, len_bytes vs len_chars.