Add support for Apple Pay JS SDK

Problem/Motivation

Braintree supports Apple Pay payment through a Mobile Safari specific JS SDK. While it's easy enough to integrate in a decoupled context without any module changes, we'd like to see this module support it in full-stack scenarios as well.

Making it work does require specific setup in your Braintree account, including activating an Apple Developer account with a valid DUNS number. It's a bit of a pain, to be honest, but we can document the steps to make it work in a guide here on d.o.

Proposed resolution

The integration guide is here: https://developers.braintreepayments.com/guides/apple-pay/client-side/ja...

Our general steps to make this work in the Braintree are:

1. Add payment gateway configuration settings to enable Apple Pay, essentially enabling the Apple Pay payment method type. We should include a link to the account configuration docs in the help text of the checkboxes element.
2. This is going to depend on Commerce 2.25 to make use of an Apple Pay payment method type plugin we'll add to core.
3. Update the payment method add form plugin to implement the build Apple Pay callbacks (building the form, responding to requests, etc.).
4. Update the payment gateway plugin to support creating Apple Pay payment methods.

We will not consider vaulting to be in scope for now. Apple has specific requirements governing when you're allowed to do this, and that work will need to be developed in conjunction with Commerce Recurring.