Unique callback handler

[JonathSpirit]

This pull request introduces significant improvements to the callback and subscription system, enhancing flexibility, clarity, and type safety. The main changes include the addition of a unique callback handler class, and a refactoring of the subscription base class to support both multi-subscriber and unique-subscriber models. These updates make it easier to manage callbacks and their lifetimes, and to distinguish between single and multiple subscriber scenarios.

Callback System Enhancements:

• Updated CallbackHandler to use CallbackStaticHelpers for callback creation, simplifying the implementation and making callback addition more consistent. (includes/FastEngine/C_callback.hpp, includes/FastEngine/C_callback.inl) [1] [2] [3]

Unique Callback Handler:

• Introduced UniqueCallbackHandler, a class designed for scenarios where only one callback is needed. This class provides methods to set, remove, and call a single callback, and manages its subscriber explicitly. (includes/FastEngine/C_callback.hpp, includes/FastEngine/C_callback.inl) [1] [2]
• Updated usage in client code to use setLambda instead of addLambda for unique callback scenarios, reflecting the new API. (examples/clientServerLifeSimulator_004/server/main.cpp)

Subscription System Refactor:

• Refactored the subscription base class, introducing BaseSubscription as an abstract base, and splitting into Subscription (multi-subscriber) and UniqueSubscription (single-subscriber) classes. This clarifies the intent and usage of each subscription type and improves type safety. (includes/FastEngine/C_subscription.hpp) [1] [2] [3] [4]

These changes collectively modernize the callback and subscription infrastructure, making it easier for developers to implement and maintain callback-based features with clear subscriber management.