[w3c/payment-request] Make billing address optional (#773)

Looks like appending a billing address to a payment credential is not a common idea all over the world. In Japan we usually do not need one for credit card authorization.
- Some merchants don't want to take additional sensitive data which is not required.
- Credit card data usually don't come with a billing address in many cases (Chrome for example). Adding one is an extra step for easy checkouts.

I would like to propose making billing address optional by adding a new param something like:

```
dictionary PaymentOptions {
  boolean requestPayerName = false;
  boolean requestPayerEmail = false;
  boolean requestPayerPhone = false;
  boolean requestShipping = false;
  PaymentShippingType shippingType = "shipping";
  boolean requestBillingAddress = false; // New!
};
```


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https://github.com/w3c/payment-request/issues/773