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.c

@@ -0,0 +1,49 @@
+#include "test_relay_chn_notify_common.h"
+
+listener_callback_info_t listener1_info;
+listener_callback_info_t listener2_info;
+
+// --- Globals for Advanced Tests ---
+SemaphoreHandle_t blocking_listener_sem = NULL;
+SemaphoreHandle_t log_check_sem = NULL;
+volatile int blocking_listener_call_count = 0;
+vprintf_like_t original_vprintf = NULL;
+
+// --- Listener Test Helper Functions ---
+
+// Clear the memory from possible garbage values
+void reset_listener_info(listener_callback_info_t* info) {
+    memset(info, 0, sizeof(listener_callback_info_t));
+}
+
+void test_listener_1(uint8_t chn_id, relay_chn_state_t old_state, relay_chn_state_t new_state) {
+    listener1_info.chn_id = chn_id;
+    listener1_info.old_state = old_state;
+    listener1_info.new_state = new_state;
+    listener1_info.call_count++;
+}
+
+void test_listener_2(uint8_t chn_id, relay_chn_state_t old_state, relay_chn_state_t new_state) {
+    listener2_info.chn_id = chn_id;
+    listener2_info.old_state = old_state;
+    listener2_info.new_state = new_state;
+    listener2_info.call_count++;
+}
+
+void blocking_listener(uint8_t chn_id, relay_chn_state_t old_state, relay_chn_state_t new_state) {
+    blocking_listener_call_count++;
+    // Block until the main test task unblocks us
+    xSemaphoreTake(blocking_listener_sem, portMAX_DELAY);
+}
+
+int log_check_vprintf(const char *format, va_list args) {
+    // Buffer to hold the formatted log message
+    char buffer[256];
+    vsnprintf(buffer, sizeof(buffer), format, args);
+
+    if (strstr(buffer, "Notify queue is full")) {
+        xSemaphoreGive(log_check_sem);
+    }
+
+    return original_vprintf(format, args);
+}