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

test_apps/main/test_relay_chn_notify_common.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include "test_common.h"
+#include "freertos/semphr.h"
+#include <stdarg.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// This is defined in the source file, we redefine it here for the test.
+// The test build must have the same CONFIG_RELAY_CHN_COUNT.
+#define TEST_RELAY_CHN_NOTIFY_QUEUE_LEN  (16 + CONFIG_RELAY_CHN_COUNT * 4)
+
+// --- Listener Test Globals ---
+typedef struct {
+    uint8_t chn_id;
+    relay_chn_state_t old_state;
+    relay_chn_state_t new_state;
+    int call_count;
+} listener_callback_info_t;
+
+// --- Listener callback infos to be defined ---
+extern listener_callback_info_t listener1_info;
+extern listener_callback_info_t listener2_info;
+
+// --- Globals for Advanced Tests ---
+extern SemaphoreHandle_t blocking_listener_sem;
+extern SemaphoreHandle_t log_check_sem;
+extern volatile int blocking_listener_call_count;
+extern vprintf_like_t original_vprintf;
+
+
+// --- Listener Test Helper Functions ---
+
+// Clear the memory from possible garbage values
+void reset_listener_info(listener_callback_info_t* info);
+
+// State listeners for notify module testing
+void test_listener_1(uint8_t chn_id, relay_chn_state_t old_state, relay_chn_state_t new_state);
+void test_listener_2(uint8_t chn_id, relay_chn_state_t old_state, relay_chn_state_t new_state);
+void blocking_listener(uint8_t chn_id, relay_chn_state_t old_state, relay_chn_state_t new_state);
+
+int log_check_vprintf(const char *format, va_list args);
+
+#ifdef __cplusplus
+}
+#endif