[Feature/Improvement] Workouts with different exercise and rest durations

**Purpose:**
As a user, I would like to do some workouts that are not "symmetric", i.e., where the duration of Exercise N could be different from the one of Exercise M. The same can be valid also for the rest times.

**More details:**
Consider this workout: at the moment, I am unable to create something this. Maybe I could "play" with the rest/repetitions to simulate this, but it would be a workaround:
30 seconds squats - 30 seconds rest - 60 seconds plank - 45 seconds rest - 20 seconds jump with rope - 10 seconds rest.

**Acceptance Criteria:**
I can customize the duration of specific exercises or rests in a workout.

Thank you very much for this app! Excellent job :)