Save customer payment methods to platform for future purchases

[dechov]

Support saving customers' payment method references in the store, choosing among them at checkout, and managing them.

Look into integrating with WooCommerce core's accommodations for "token" management [wiki], and the token class in particular [doc] (not to be confused with tokenization of card details, tracked in #11).

(Not currently in v1 scope)

[thenbrent]

We should save customers and their payment methods on the platform account so that (eventually) after a customer makes a purchase from one store, we can make it so that they don’t need to re-enter their details to purchase from another store. Stripe provides docs here for sharing customers across accounts. I've created a server issue to track this: 256-gh-woocommerce-payments-server.

Importantly, we are also contractually obliged with Stripe to save customers on connected accounts as we are currently doing.

[allendav]

I hadn't found this issue when I wrote #682 and #683 - so I've decided to keep them all and tweak this issue title to specify the platform orientation @thenbrent just mentioned

[vbelolapotkov]

Not sure if the issues is still relevant. Asked internally here: p1635781961051200-slack-C022WMN88KG

[vbelolapotkov]

Closing as discussed in the slack thread above.