Add notification system for relay channel state changes

- Introduced a new notification module to handle state change listeners.
- Added functions to register and unregister listeners for relay channel state changes.
- Implemented a queue-based system to manage notifications and listener callbacks.
- Updated core relay channel logic to utilize the new notification system.
- Removed old listener management code from relay channel core.
- Refactored the former listener tests to notify tests and added tests for the notification system, including handling of multiple listeners and queue overflow scenarios.
- Updated CMakeLists.txt to include new source files and headers for the notification module.
- Revised README.md to include warnings about callback execution context and performance considerations.

Refs #1096, #1085 and closes #1097

README.md

@@ -200,6 +200,9 @@ relay_chn_flip_direction_all();
 
 ### 3. Monitor channel state
 
+> [!WARNING]
+> Listener callbacks are executed from the context of the notification dispatcher task. To ensure system responsiveness and prevent event loss, callbacks must be lightweight and non-blocking. Avoid any long-running operations or functions that may block, such as `vTaskDelay()` or semaphore takes with long timeouts, inside the callback.
+
 For single mode:
 
 ```c