Change the format of run and ramp time

The run-time and ramp-time are configured through the config file using an int that is treated as a duration in minutes. As part of CI we want to do short runs of spectroperf, e.g 30 seconds and to support this we want to change the run and ramp time to be strings and the user can specify the value and the unit.

Start by locating the config parsing and runtime handling for run-time and ramp-time, then check how CI invokes spectroperf. Update the configuration behavior so these values accept a numeric value with a unit, and verify that a short duration such as 30 seconds works in CI.

Written by the indexing model from the issue text.