Add option to set different types of exercises

[spar34vi]

Is your feature request related to a problem? Please describe.
Not every exercise consists in n° of reps × n°of sets.

Describe the solution you'd like
Adding the option to set exercise based on time (e.g. 3 mins of cardio), distance (2km on the treadmill) etc.

Describe alternatives you've considered
Writing on the notes

Additional context
Something like the app FitNotes (screenshot down below)

[LiamMorrow]

This one does come up a fair amount. (#350 #322).

Thanks for the link to fitNotes, it helps give me ideas of what kinds of things need to be tracked. I haven't done it so far largely because I have never needed it and therefore don't have any idea what a GOOD implementation would be like.

The demand is obviously there, so I'll chew on it for a while