[w3c/payment-handler] Add support for communicating billing address to merchant (e.g., for VAT computation) (#314)

This issue is the payment handler complement to Payment Request API issue 27:
 https://github.com/w3c/payment-request/issues/27

Example flow:

* The user selects a payment handler.
* Within the payment handler, the user selects a payment instrument with associated billing address.
* The payment handler informs the browser that the user has selected a billing address. The payment handler waits for a response (read: promise to resolve) that may update the total. The payment handler will not complete the transaction until hearing back from the merchant.
* The browser fires the onpaymentmethodchange event to let the merchant know the billing address has changed.
* The merchant responds with a (possibly null) update of the total.
* The browser gives the (possibly updated) total back to the payment handler.
* The payment handler displays the updated total and allows the user to proceed.

Let's keep an eye on this pull request:
 https://github.com/w3c/payment-request/pull/749

And this one:
 https://github.com/w3c/payment-method-basic-card/pull/53


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https://github.com/w3c/payment-handler/issues/314