feat: addProductsToArticleGroup API call

[roydejong]

This adds support for the addProductsToArticleGroup API call.

Example:

<?php

/**
 * @var $client MplusQAPIclient
 */
$response = $client->addProductsToArticleGroup(1234, [1, 2, 3, 4]);

var_dump($response);
// boolean true