[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 

Do we really need an ivar for this?