[Go] Revert commit 5e88857 to undelete the 'callback' and 'extend' examples.

The 'callback' and 'extend' examples were presumed to be obsoleted by the new
'director' example.  The examples are helpful though to have similar examples
across target languages and hence the commit @5e88857 which removed these
examples got reverted.

Examples/go/callback/example.i

@@ -0,0 +1,11 @@
+/* File : example.i */
+%module(directors="1") example
+%{
+#include "example.h"
+%}
+
+/* turn on director wrapping Callback */
+%feature("director") Callback;
+
+%include "example.h"
+