text: Title: Add forward compatible `start_time`, `end_time` and `elapsed_time` propertys to result objects
  Body: The plan is to enhance performance of getting and processing timestamps in RF 7.0 (#4258). Part of that is internally representing start and end times as `datetime` objects and elapsed time as a `timedelta`. The old `starttime`, `endtime` and `elapsed` time attributes will be preserved for backwards compatibility reasons, and new `start_time`, `end_time` and `elapsed_time` attributes added.
  
  For forward compatibility reasons we should add `start_time`, `end_time` and `elapsed_time` already in RF 6.1. They can be propetys that get their values from the old attributes. In RF 7 we can then change them to be the "real" attributes and make the old ones propertys.