feat(mosq): Add support for on-message callback

components/mosquitto/port/include/mosq_broker.h

@@ -9,6 +9,7 @@
 
 struct mosquitto__config;
 
+typedef void (*mosq_message_cb_t)(char *client, char *topic, char *data, int len, int qos, int retain);
 /**
  * @brief Mosquitto configuration structure
  *
@@ -24,6 +25,10 @@ struct mosq_broker_config {
                                      * You can open the respective docs with this idf.py command:
                                      * `idf.py docs -sp api-reference/protocols/esp_tls.html`
                                      */
+    void (*handle_message_cb)(char *client, char *topic, char *data, int len, int qos, int retain); /*!<
+                                     * On message callback. If configured, user function is called
+                                     * whenever mosquitto processes a message.
+                                     */
 };
 
 /**