Adds support for fetching BigCommerce Products by SKU. Including via shortcode

[emiliorcueto]

Adds support for fetching BigCommerce Products by SKU.

• Adds static methods by_product_sku and query to the BigCommerce\Post_Types\Product class
• Add default attribute of sku to the Product_Components class implementing the bc-component shortcode

Documentation

Historically, one would only be able to fetch a product with a BigCommerce Product ID
E.g. $product = \BigCommerce\Post_Types\Product\Product::by_product_id( $product_id );

This pull request adds the ability to fetch of a product in a very similar fashion by BigCommerce Product SKU
E.g. $product = \BigCommerce\Post_Types\Product\Product::by_product_sku( $product_sku );

Additionally, the bc-component shortcode attributes have been updated to support fetching by BigCommerce Product SKU as well.
E.g. echo do_shortcode('[bc-component sku="' . $product_sku . '" type="add_to_cart"]');