[w3c/payment-request] wip: allow show to take optional promise (#668)

Changes `show()` to take an optional promise that resolves with a `PaymentDetailsUpdate`:

```
Promise&<PaymentResponse> show(optional Promise<PaymentDetailsUpdate> detailsPromise);
```

We can then reuse the machinery to perform an update, but splitting it out into its own algorithm
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/payment-request/pull/668

-- Commit Summary --

  * wip: allow show to take optional promise

-- File Changes --

    M index.html (645)

-- Patch Links --

https://github.com/w3c/payment-request/pull/668.patch
https://github.com/w3c/payment-request/pull/668.diff

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