Implemented shared state, simplified init phase, added more join points into context

[EnvOut]

Based on the feedback provided for uactor, I’ve rewritten part of the library. Key updates include:

1. Simplified actor initialization:

   • Replaced macros with regular method calls.
   • Streamlined the initialization process.

2. Added more join points in context/actor for service logic (e.g., metrics, error handling).

3. Introduced an alias for references using an unbounded MPSC channel:

   • Actor1Ref<mpsc::Sender<Actor1Msg>> -> Actor1MpscRef.

4. Finalized the shared state feature:

   • An actor can now have shared state that is embedded within its reference.
   • Access the state via:

     actor1_ref.state()

   • Use case example in arbiter/upstream:

     actor1_ref.state().downstreams_count()