Look up product by sku

Extending products REST API GET request to lookup a product by sku. Register new route @ /products/sku/<product sku> and call get_product_by_sku(). Iterate over product collection, match SKU, validate related product ID, and return.

Request

    https://www.example.com/products/sku/1111

Return

    {
        products: [
            {
                title: "Charleston, Then and Now hardcover book",
                id: 57,
                created_at: "2014-10-06T17:27:48Z",
                updated_at: "2014-11-04T19:54:13Z",
                type: "simple", 
                ...

Conforms to current error type(s) if no SKU returned
 
    errors: [
        {
            code: "woocommerce_api_invalid_sku",
            message: "Error - sku not found"
        }
    ]