Signature example for the message handler provider callable.
ICanBoogie\MessageBus\SimpleMessageHandlerProvider
public ICanBoogie\MessageBus\MessageHandler|callable
ICanBoogie\MessageBus\MessageHandler
__invoke( ICanBoogie\MessageBus\Message $message )
ICanBoogie\MessageBus\Message
ICanBoogie\MessageBus\NoHandlerForMessage