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/CMakeLists.txt

@@ -1,5 +1,6 @@
 # === These files must be included in any case ===
 set(srcs "test_common.c"
+         "test_relay_chn_notify_common.c"
          "test_app_main.c")
 
 set(incdirs ".")
@@ -7,10 +8,10 @@ set(incdirs ".")
 # === Selective compilation based on channel count ===
 if(CONFIG_RELAY_CHN_COUNT GREATER 1)
     list(APPEND srcs "test_relay_chn_core_multi.c"
-                     "test_relay_chn_listener_multi.c")
+                     "test_relay_chn_notify_multi.c")
 else()
     list(APPEND srcs "test_relay_chn_core_single.c"
-                     "test_relay_chn_listener_single.c")
+                     "test_relay_chn_notify_single.c")
 endif()
 
 if(CONFIG_RELAY_CHN_ENABLE_TILTING)