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

private_include/relay_chn_notify.h

@@ -0,0 +1,51 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Kozmotronik Tech
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include "esp_err.h"
+#include <stdint.h>
+#include "relay_chn_types.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief Init the notify module.
+ * 
+ * @return
+ *      - ESP_OK: Success
+ *      - ESP_ERR_NO_MEM: Not enough memory to create notify queue
+ */
+esp_err_t relay_chn_notify_init(void);
+
+/**
+ * @brief Deinit the notify module.
+ * 
+ * This function cleans up resources used by the notify module.
+ */
+void relay_chn_notify_deinit(void);
+
+/**
+ * @brief Notify all registered listeners about a state change.
+ *
+ * This function sends a state change event to an internal queue, which will then
+ * be processed by a dedicated task to notify all registered listeners. This
+ * function is typically called internally by the relay channel core logic.
+ *
+ * @param chn_id The ID of the relay channel whose state has changed.
+ * @param old_state The previous state of the relay channel.
+ * @param new_state The new state of the relay channel.
+ */
+esp_err_t relay_chn_notify_state_change(uint8_t chn_id,
+                                        relay_chn_state_t old_state,
+                                        relay_chn_state_t new_state);
+
+#ifdef __cplusplus
+}
+#endif
+