Feature: Apple Pay

[ChaseWiseman]

This PR adds handlers to support Apple Pay on the checkout, cart, and single product pages.

Gateway Implementation

1. Declare support for any gateway extending SV_WC_Payment_Gateway by using self::FEATURE_APPLE_PAY
2. Override the SV_WC_Payment_Gateway::get_order_for_apple_pay() method to add any data necessary for processing the transaction via the gateway's API. See SV_WC_Payment_Gateway_Apple_Pay_Payment_Response for returned data.

By default, the feature is inactive and will need to be activated by a filter:

add_filter( 'wc_payment_gateway_{gateway_id}_activate_apple_pay', '__return_true' );

This makes the WooCommerce -> Settings -> Checkout -> Apple Pay settings section available for configuration.

The classes and JS handle the entire Apple Pay flow, and when successful simply calls SV_WC_Payment_Gateway::process_payment(). From there the gateway handles everything like a normal transaction, and it's up to the gateway to set the appropriate API params with the data it sets in SV_WC_Payment_Gateway::get_order_for_apple_pay().

Limitations

• No subscription or pre-order (pay on release) transactions. If either exists in the cart, Apple Pay is unavailable.
• On the single product page, Apple Pay is only available for simple products. Additionally, the Apple Pay button is treated as a "Buy Now" button to quickly buy a single item, so the Quantity add-to-cart input is not a factor.
• Improvements are being logged in the GH project: https://github.com/skyverge/wc-plugin-framework/projects/3

[bekarice]

@ChaseWiseman looks great, and SONP works well 🎉 really nice stuff here, the taxes and shipping handling in the Apple Pay modal is 💯

One fix: the accepted currencies is failing, I think perhaps due to the supported / enabled Apple Pay gateway check:

As a final note, one thing we may want to disable Apple Pay on Membership-granting products like Subscriptions (not needed before deploy, we can follow with an update for AIM / CIM).

The code here could likely benefit from a helper -- something like wc_memberships_product_grants_membership_access( $product_id ) (bad name, I know) to return an array of plans the product grants access to, which could also be used within Memberships itself -- cc @unfulvio

That way we could bail on the product / cart / checkout. We could also consider making that Memberships WC_Memberships_Checkout::force_registration() check public so the FW could check that in the cart / checkout and bail if it's false. Something for the first feature release anyway.

[maxrice]

💯 this looks awesome, great work @ChaseWiseman! Really well-organized and the CoffeeScript is aces.