[FEATURE] - `Unsubscribe` function and message in `PubSub` actor in `kameo_actors`

[yawor]

The PubSub actor currently unsubscribes the subscriber only when the subscriber is not running or is stopped. It would be desirable to have the ability to unsubscribe a subscriber which is still active.

This could be achieved by adding unsubscribe function and Unsubscribe message.

[tqwewe]

This sounds reasonable, I'd be open to supporting this in a PR