code: public Javalin addHandler(@NotNull HandlerType httpMethod, @NotNull String path, @NotNull Handler handler) {
          return addHandler(httpMethod, path, handler, new HashSet<>()); // no roles set for this route (open to everyone with default access manager)
      }
docs: Adds a request handler for the specified handlerType and path to the instance.
  This is the method that all the verb-methods (get/post/put/etc) call.
  
  @see <a href="https://javalin.io/documentation#handlers">Handlers in docs</a>