text: Title: Add generation time from output.xml to `Result` object
  Body: The `output.xml` has  `<robot generated="year-month-dayTHH:MM:SS.milliseconds" …>` in the xml. It would be handy to expose the `generated` attribute from the RF result package. We have a need to  use the `generated` attribute when reading test result from multiple runs in CI and plotting the results in time series manner.  