Add history level persisting process instance variable without full activity history

**Is your feature request related to a problem? Please describe.**

I've been using `FULL` history level so far. There's a lot of redundant data that I don't need (and the tables get huge - tens of GBs) - mainly activity history (`act_hi_actinst`) and variable changes (`act_hi_detail`). I can easily get rid of the second one by using history level `ACTIVITY`.

Unfortunately, the activity's history is coupled with the variable's history. I really need the variables. I'm surprised that the `INSTANCE` history level is not including them. I really can't see what's the use of persisting instances without the variables.

**Describe the solution you'd like**

Ideally from my PoV the `INSTANCE` history level should also persist variables. If this is not a bug and it really makes sense for some users, then another level achieving the same thing works as well.

**Describe alternatives you've considered**

I'm considering background jobs clearing the `act_hi_actinst` table periodically.

**Additional context**

What further makes it worse is that:
- I'm having long-running processes with polling which further generates more activity history (by looping on the diagram).
- The variable updates are stored even when not changing anything (i.e. deleting not existing variable or setting the same value again).

Inizia tracciando come i livelli di cronologia INSTANCE, ACTIVITY e FULL regolano la persistenza delle variabili dell’istanza di processo, prestando particolare attenzione a act_hi_actinst e act_hi_detail. Il lavoro è completato quando è stato definito e implementato un comportamento del livello di cronologia che preservi le variabili richieste senza richiedere la cronologia completa delle attività e quando i suoi effetti sono documentati e testati.

Scritto dal modello di indicizzazione a partire dal testo della issue.