code: public Javalin attribute(Class clazz, Object obj) {
          config.inner.appAttributes.put(clazz, obj);
          return this;
      }
docs: Registers an attribute on the instance.
  Instance is available on the {@link Context} through {@link Context#appAttribute}.
  Ex: app.attribute(MyExt.class, myExtInstance())
  The method must be called before {@link Javalin#start()}.