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

include/relay_chn_adapter.h

@@ -13,6 +13,36 @@
 extern "C" {
 #endif
 
+/**
+ * @brief Register a channel state change listener.
+ * 
+ * @param listener A function that implements relay_chn_state_listener_t interface.
+ *
+ * @return
+ *     - ESP_OK: Success
+ *     - ESP_ERR_INVALID_ARG: Invalid argument
+ *     - ESP_ERR_NO_MEM: No enough memory
+ *     - ESP_FAIL: General failure
+ */
+extern esp_err_t relay_chn_notify_add_listener(relay_chn_state_listener_t listener);
+
+/**
+ * @brief Unregister a channel state change listener.
+ * 
+ * @param listener A function that implements relay_chn_state_listener_t interface.
+ */
+extern void relay_chn_notify_remove_listener(relay_chn_state_listener_t listener);
+
+static inline esp_err_t relay_chn_register_listener(relay_chn_state_listener_t listener)
+{
+    return relay_chn_notify_add_listener(listener);
+}
+
+static inline void relay_chn_unregister_listener(relay_chn_state_listener_t listener)
+{
+    relay_chn_notify_remove_listener(listener);
+}
+
 #if CONFIG_RELAY_CHN_COUNT > 1
 /**
  * @brief Get the current state of a relay channel.