[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 thought about that, but unless my understanding of `store`'s purpose is off, I'm not sure that it *really* makes sense here. The Apple Pay tokens expire after 24 hours; they're meant to be throwaway data for a single transaction, so does it didn't seem to make sense to put that logic into `store`. 
Does that make sense?