refactor: update type of `actor` field in the session

[chungquantin]

actor field in Session is no longer used in the main logic of the session with the introduction of Revive. Reason for that is all internal logic of the drink library requires the RuntimeOrigin type. Hence, the actor field is only used in the get_actor method. However, as the method is used by the developers to debug the current actor, it would be best to return AccountId instead of RuntimeOrigin.