Activity should be able to be called outside of subscriber

### Description

An Activity should be able to handle being called without being a subscriber callback.
That is, you should be able to call an activity.

Example:
```
class MyActivity(Activity):
pass
activity = MyActivity(...)
activity()
# Stuff happens
```

### Motivation
Sometimes an activity can be triggered without listening on events, for example when creating a tool that sends activity triggered,started,finished.

### Exemplification

Activities should be allowed to be used as activities started and not just activities triggered.

### Benefits

Ease of use.

### Possible Drawbacks

Maybe pinning too much functionality to the activity object.

Comienza localizando la implementación de Activity y la ruta del subscriber callback en la Python library. Determina cómo deben relacionarse las llamadas directas con los eventos triggered, started y finished, y verifica después que el comportamiento existente de los subscriber se mantenga intacto y que la invocación directa esté cubierta por pruebas.

Escrito por el modelo de indexación a partir del texto del issue.