WIP - needs Arbab's Auth.net XML jazz for ApplePay

* Adds Cryptogram support
* See ActiveMerchant::Billing::AuthorizeNetGateway#add_apple_pay

This is based off of @ntalbott ’s suggestions in https://github.com/Shopify/active_merchant/pull/1433

@bizla Could you please add your magic XML stuff + your tests here?

CC @Shopify/active-merchant 
I kinda hate seeing `is_a?` since it suggests a missing ducktype but it does make things pretty simple in this case.