Implementing remaining PKPaymentAuthorizationViewControllerDelegate methods

I haven't written any tests since this is basically just a passthrough of the delegate calls. We found it useful since we're calculating tax when a user selects an address on the Apple Pay sheet.

Not sure if defining methods in `BTPaymentMethodCreationDelegate.h` as `@optional` is the best way to go. Any suggestions?
Maybe a technique that would align with our goals for Braintree-iOS more would be to expose a `delegate` property. This way callers can have full access to the `PKPaymentAuthorizationViewControllerDelegate` callbacks while `BTPaymentProvider` is authorizing Apple Pay.