allow setting of cusom length lwt msg

include/mqtt_client.h

@@ -67,6 +67,7 @@ typedef struct {
     char lwt_msg[MQTT_MAX_LWT_MSG];
     int lwt_qos;
     int lwt_retain;
+    int lwt_msg_len;
     int disable_clean_session;
     int keepalive;
     bool disable_auto_reconnect;

mqtt_client.c

@@ -127,7 +127,10 @@ static esp_err_t esp_mqtt_set_config(esp_mqtt_client_handle_t client, const esp_
 
     if (config->lwt_msg[0]) {
         client->connect_info.will_message = strdup(config->lwt_msg);
-        client->connect_info.will_length = strlen(config->lwt_msg);
+        if (config->lwt_msg_len > 0)
+        		client->connect_info.will_length = config->lwt_msg_len;
+        else
+    			client->connect_info.will_length = strlen(config->lwt_msg);
     }
 
     client->connect_info.will_qos = config->lwt_qos;