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

CMakeLists.txt

@@ -3,7 +3,8 @@ set(priv_include_dirs "private_include")
 
 set(srcs "src/relay_chn_core.c"
          "src/relay_chn_output.c"
-         "src/relay_chn_run_info.c")
+         "src/relay_chn_run_info.c"
+         "src/relay_chn_notify.c")
 
 if(CONFIG_RELAY_CHN_ENABLE_TILTING)
     list(APPEND srcs "src/relay_chn_tilt.c")