Function process_spreadsheet_async 

pub async fn process_spreadsheet_async(
    opts: &OptionSet,
    save_func: Box<dyn Fn(IndexMap<String, Value>) -> Result<(), GenericError> + Send + Sync>,
    out_ref: Option<&str>,
) -> Result<ResultSet, GenericError>

Output the result set with deferred row saving and optional output reference