Add some Http3 skeleton

src/Http3Context.h

@@ -0,0 +1,17 @@
+namespace uWS {
+    struct Http3Context {
+        static Http3Context *create(us_loop_t *loop, us_socket_context_options_t options) {
+            return nullptr;
+
+            // call init here after setting the ext to Http3ContextData
+        }
+
+        void init() {
+            // set all callbacks here
+        }
+
+        void onHttp() {
+            // modifies the router we own as part of Http3ContextData, used in callbacks set in init
+        }
+    };
+}