* @param function A function to transform the result of the input future to the result of the
   *     output future
   * @param executor Executor to run the function in.
   * @return A future that holds result of the function (if the input succeeded) or the original
   *     input's failure (if not)
   * @since 19.0 (in 11.0 as {@code transform})
   */
  public static <I extends @Nullable Object, O extends @Nullable Object>