Support vaulted ApplePay

[balexand]

This adds support for vaulted Apple Pay payment methods. Braintree.PaymentMethod.create/2 can now be called with an Apple Pay nonce. Also, the Braintree.Customer struct now includes an apple_pay_cards field.

I was not able to get the complete integration test suite to pass, but the integration tests in test/integration/payment_method_test.exs worked for me. I added an integration test to this file covering Braintree.PaymentMethod.create/2.

Please let me know if there's anything that you'd like changed.

Thanks ✨

[balexand]

The docs for ApplePayCard can be found here: https://developers.braintreepayments.com/reference/response/apple-pay-card/ruby

I've only included documented fields in the Braintree.ApplePayCard struct. The exception to this is billing_address, which is missing from Braintree's docs but is present in the JSON returned by Braintree.

[sorentwo]

Thanks for the improvements @balexand. Everything looks great. Don't sweat the full test suite, from what sources at Braintree have told me that is an issue that they face as well. Rate limiting and integration testing is a poor combination.