Shipping-class mandatory

@matthieuh, our plugin by default assigns the vendo’r shipping class to the product in the frontend Add Product Page.

However, the backed add product page comes from WooCommerce, and WooCommerce doesn’t set any shipping class by default. So, if you want to set shipping class of that vendor in the backend too, you have to do custom code.

For this please check our code for frontend and see how we are adding this flow in frontend from here – https://github.com/dualcube/dc-woocommerce-multi-vendor/blob/master/templates/vendor-dashboard/product-manager/add-product.php

Apply the same logic in the backend, by checking if the user is dc_vendor.