Added product/images method support

[saranyan]

No description provided.

[maetl]

Any reason why these methods are named as Image rather than ProductImage?

getProductImages($productId)
createProductImage($productId, $image)
# etc...

What would be the best approach here?

Product images should already be supported in the API client via the following syntax:

<?php

use Bigcommerce\Api\Resources\ProductImage;

$image = new ProductImage();
$image->product_id = 1;
$image->image_file = "/picture.png";
$image-> description = "Picture";
$image->create();

[maetl]

Merging now—a subsequent update will make the API signatures symmetric, as described above.