AsyncWiFiManagerParameter add boolean parameter

[anubisg1]

I'm try to add a boolean parameter, so that if it's enabled, other parameters would be displayed in the captive portal.

specifically something on this line:

[ ] enable MQTT

if the button is selected, then

    AsyncWiFiManagerParameter custom_mqtt_server("server", "MQTT server", MQTT_SERVER, 40);
    AsyncWiFiManagerParameter custom_mqtt_port("port", "MQTT port", MQTT_PORT, 5);
    AsyncWiFiManagerParameter custom_mqtt_username("username", "MQTT username", MQTT_USER, 100);
    AsyncWiFiManagerParameter custom_mqtt_password("password", "MQTT password", MQTT_PASSWORD, 100);

would be displayed as UI parameters to be filled in by the user