Add Adyen plugin

[Sajgon27]

Integrates sylius/adyen-plugin (with its sylius/refund-plugin): registers bundles, config/routes imports, refund gateway support, and admin/shop assets (@adyen/adyen-web). ProductVariant now implements CommodityCodeAwareInterface.

[GSadee]

I would suggest to exclude the failing behat scenarios, as they will not work with the RefundPlugin installed, you can install friends-of-behat/exclude-specifications-extension and configure it in behat.yml.dist:

FriendsOfBehat\ExcludeSpecificationsExtension:
    features:
    - "vendor/sylius/sylius/features/admin/order/managing_orders/handling_order_payment/refunding_order_payment.feature"