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.

src/relay_chn_ctl_multi.c

@@ -5,6 +5,7 @@
  */
 
 #include "esp_check.h"
+#include "relay_chn.h"
 #include "relay_chn_priv_types.h"
 #include "relay_chn_core.h"
 #include "relay_chn_ctl.h"
@@ -92,8 +93,8 @@ esp_err_t relay_chn_ctl_get_state_all(relay_chn_state_t *states)
 char *relay_chn_ctl_get_state_str(uint8_t chn_id)
 {
     return relay_chn_is_channel_id_valid(chn_id)
-           ? relay_chn_state_str(s_chn_ctls[chn_id].state)
-           : relay_chn_state_str(RELAY_CHN_STATE_UNDEFINED);
+           ? relay_chn_state_to_str(s_chn_ctls[chn_id].state)
+           : relay_chn_state_to_str(RELAY_CHN_STATE_UNDEFINED);
 }