[Stripe] Add Apple Pay Payment Token exchanging

Stripe requires that inbound Apple Pay Payment Token data be exchanged for a Stripe payment token for use with their money-transaction endpoints, like any other card data you wish to tokenize when using Stripe. 

This adds the necessary POST to the Stripe Tokens API when Apple Pay Payment Tokens are involved.

@j-mutter @jnormore /cc @Shopify/active-merchant @ntalbott 

I'm not a fan of these `assert_instance_of Response` checks; they're vestigial/legacy and unnecessary IMO.