code: public ItemsResponse getItemsByExternalId(int appId, String externalId) {
  		return getItems(appId, null, null, null, null,
  				new FilterByValue<String>(new ExternalIdFilterBy(), externalId));
  	}
docs: Utility method to get items matching an external id
  
  @param appId
  The id of the app
  @param externalId
  The external id
  @return The items matching the app and external id