Add NVS support for relay channel config persistence

- Introduced NVS configuration options in Kconfig.
- Implemented NVS initialization and deinitialization in relay_chn_core.
- Added functions for storing and retrieving relay channel direction and tilt sensitivity in NVS.
- Updated relay_chn_tilt and relay_chn_output to utilize NVS for state management.
- Created relay_chn_nvs.c and relay_chn_nvs.h for NVS-related functionalities.

Closes #1074.

Kconfig

@@ -26,4 +26,39 @@ menu "Relay Channel Driver Configuration"
             at a time. Tilting is specifically designed for controlling some
             types of curtains that need to be adjusted to let enter specific
             amount of day light.
+    
+    config RELAY_CHN_ENABLE_NVS
+        bool "Enable persistent NVS storage for relay channel"
+        default n
+        help
+            If enabled, relay channel configuration will be stored in NVS.
+
+endmenu
+
+menu "Relay Channel NVS Storage Configuration"
+    depends on RELAY_CHN_ENABLE_NVS
+
+    config RELAY_CHN_NVS_NAMESPACE
+        string "NVS namespace for relay channel storage"
+        default "relay_chn"
+        help
+            The NVS namespace used for storing relay channel configuration.
+            This should be unique to avoid conflicts with other components.
+
+    config RELAY_CHN_NVS_CUSTOM_PARTITION
+        bool "Use custom NVS partition for relay channel storage"
+        default n
+        help
+            If enabled, a custom NVS partition will be used for storing
+            relay channel configuration. If disabled, the default NVS
+            partition will be used.
+    
+    config RELAY_CHN_NVS_CUSTOM_PARTITION_NAME
+        string "Custom NVS partition name"
+        depends on RELAY_CHN_NVS_CUSTOM_PARTITION
+        default "app_data"
+        help
+            The name of the custom NVS partition used for storing relay channel
+            configuration. Make sure the name is exactly the same as label defined
+            in the relevant partition table.
 endmenu