Async State Subscriptions

[ztnel]

Implement asynchronous state model level subscription where consumers can subscribe to a commit made to a specific state model. Subscriptions are implemented by passing a callback which accepts a single parameter for the new copy of the requested state model:

def __init__(self):       
   with State() as state:
      state.subscribe(User, self._schedule)

async def _schedule(self, user: User) -> None: ...