Add virtual beginMulticast(...) stub to UDP class (#4061)

https://github.com/arduino/ArduinoCore-sam/pull/6/files

It is used here:
https://github.com/arduino-libraries/ArduinoMDNS/blob/master/MDNS.cpp#L150

Author: takeru

Diff for: cores/esp32/Udp.h

@@ -43,6 +43,7 @@ class UDP: public Stream
 
 public:
     virtual uint8_t begin(uint16_t) =0;	// initialize, start listening on specified port. Returns 1 if successful, 0 if there are no sockets available to use
+    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
     virtual void stop() =0;  // Finish with the UDP socket
 
     // Sending UDP packets