code: public static Iterable<BoxUser.Info> getAppUsersByExternalAppUserID(final BoxAPIConnection api,
             final String externalAppUserId, final String... fields) {
          return getUsersInfoForType(api, null, null, externalAppUserId, fields);
      }
docs: Gets any app users that has an exact match with the externalAppUserId term.
  @param api                 the API connection to be used when retrieving the users.
  @param externalAppUserId    the external app user id that has been set for app user
  @param fields               the fields to retrieve. Leave this out for the standard fields.
  @return an iterable containing users matching the given email