Add UDP transmitter example (#306)

Co-authored-by: Andrew McDonnell <bugs@andrewmcdonnell.net>

pico_w/udp_beacon/lwipopts.h

@@ -0,0 +1,10 @@
+#ifndef _LWIPOPTS_H
+#define _LWIPOPTS_H
+
+// Generally you would define your own explicit list of lwIP options
+// (see https://www.nongnu.org/lwip/2_1_x/group__lwip__opts.html)
+//
+// This example uses a common include to avoid repetition
+#include "lwipopts_examples_common.h"
+
+#endif