Support `FixedSizedListArray` for `length` kernel

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

While implement unnest in DataFusion for FixedSizeListArray (see apache/datafusion#6903) it turns out there is no support for FixedSizeListArray in the length kernel

https://docs.rs/arrow/latest/arrow/compute/kernels/length/fn.length.html

Describe the solution you'd like
While it sounds silly, I would like to Support FixedSizedListArray for length kernel

Specifically, it would be nice to get the NULLs handled correctly even though all the values will of course be the same (fixed!) size lenth

Here is a reproducer:

fn main() {
    // Construct a value array
    let value_data = ArrayData::builder(DataType::Int32)
        .len(9)
        .add_buffer(Buffer::from_slice_ref(&[0, 1, 2, 3, 4, 5, 6, 7, 8]))
        .build()
        .unwrap();
    let list_data_type = DataType::FixedSizeList(
        Arc::new(Field::new("item", DataType::Int32, false)),
        3,
    );
    let nulls = NullBuffer::from(vec![true, false, true]);
    let list_data = ArrayData::builder(list_data_type.clone())
        .len(3)
        .add_child_data(value_data.clone())
        .nulls(Some(nulls))
        .build()
        .unwrap();
    let list_array = FixedSizeListArray::from(list_data);


    let lengths = arrow::compute::kernels::length::length(&list_array).unwrap();
    println!("{}", pretty_format_columns("lengths", &[lengths]).unwrap());

}

It should print out [3, null, 3]

Describe alternatives you've considered

Additional context

[alamb]

I think this is a good first issue as it is well specified and has a reproducer

[Weijun-H]

willing to take this issue.