http2: initial implementation of the push callback

lib/multihandle.h

@@ -87,6 +87,10 @@ struct Curl_multi {
   curl_socket_callback socket_cb;
   void *socket_userp;
 
+  /* callback function and user data pointer for server push */
+  curl_push_callback push_cb;
+  void *push_userp;
+
   /* Hostname cache */
   struct curl_hash hostcache;