Add virtual beginMulticast(...) stub to UDP class

[lmartorella]

Hi,
It seems that (at least) the Arduino core and the ESP32 core both has the base beginMulticast virtual definition in the base UDP class:

     virtual uint8_t beginMulticast(IPAddress, uint16_t) { return 0; }  // initialize, start listening on specified multicast IP address and port. Returns 1 if successful, 0 on failure

In order to be compatible with libraries like arduino-libraries/ArduinoMDNS, I've added the same base virtual function at UDP level here too.

With these changes, the above library works on a ESP8266 like a charm.

I'm not sure if the different already existing signature of the WiFiUDP::beginMulticast should be left here or not. To be backward compatible, I've added both signatures, but not sure if the one with the different signature should be tagged as deprecated or not.

Thanks!
L