feat!: add on_message hook to Actor trait

[tqwewe]

This PR introduces a new on_message hook to the Actor trait.

Overview:

The on_message method allows actors to customize how incoming messages are handled. By default, the method processes the message immediately using the actor’s standard message handling logic (msg.handle_dyn). However, this method is designed to offer flexibility for advanced use cases where immediate handling may not be ideal. For example, developers can override on_message to:

• Buffer messages for delayed processing.
• Implement custom scheduling or prioritization logic.
• Manipulate or filter raw messages before they are handled.

Key Changes:

• New on_message Method: Handles the incoming message, with the option to override for custom processing behavior.
• Default Behavior: Processes the message using msg.handle_dyn(self, actor_ref, tx).await.
• Advanced Use Cases: Supports scenarios where buffering or deferring message handling is required.

This addition provides more control over how messages are processed, enhancing the flexibility of actor behavior in complex systems.