Push down byte_length

[blaginin]

I'm reading a big VarBinViewArray/ VarBinArray column with byte_length(col) - currently it loads the buffers for that projection. I think we can push down that expression and not actually load the data?

synthetic-format-metadata.vortex.zip

let array = vortex_session()
        .open_options()
        .open(reader.clone() as Arc<dyn VortexReadAt>)
        .await?
        .scan()?
        .with_projection(pack(
            [
                (FieldName::from("id"), get_item("id", root())),
                (
                    FieldName::from("size_bytes"),
                    get_item("size_bytes", root()),
                ),
                (
                    FieldName::from("format_metadata_bytes"),
                    byte_length(get_item("format_metadata", root())),
                ),
            ],
            Nullability::NonNullable,
        ))
        .into_array_stream()?
        .read_all()
        .await?;

[robert3005]

This could leverage the same mechanism as listlayout but right now whole array is stored in single place segment