Add state to string API function

Added a state to string public API function. There was already
a private function (`relay_chn_state_str`) that provides
this functionality. So this function has been renamed to
`relay_chn_state_to_str` and made publicly available.

Refs #1104, #1105 and closes #1108.

include/relay_chn.h

@@ -63,6 +63,14 @@ esp_err_t relay_chn_register_listener(relay_chn_state_listener_t listener);
  */
 void relay_chn_unregister_listener(relay_chn_state_listener_t listener);
 
+/**
+ * @brief Return the text presentation of an state.
+ *
+ * @param state A state with type of relay_chn_state_t.
+ * @return char* The text presentation of the state. "UNKNOWN" if the state is not known.
+ */
+char *relay_chn_state_to_str(relay_chn_state_t state);
+
 #if CONFIG_RELAY_CHN_ENABLE_TILTING
 /**
  * @brief Get the default tilting sensitivity for the relay channel.