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.

まず、Python library 内の Activity の実装と subscriber callback の経路を特定します。直接呼び出しが triggered、started、finished イベントとどのように関係すべきかを判断し、その後、既存の subscriber の動作が維持され、直接呼び出しがテストでカバーされていることを確認します。

索引モデルが issue の本文から書いたものです。