Request billing address

Click on each button in sequence from top to bottom without refreshing the page. Each button will bring up the Payment Request UI window.

When the payment sheet is presented, select a payment method (e.g., a credit card), and press "Pay".

1. When no billing address is requested, `PaymentMethodChangeEvent.methodData.billingAddress` is null.
2. When billing address is requested,`PaymentMethodChangeEvent.methodData.billingAddress` is a `PaymentAddress`.
3. Done!