[webpayments] How are cloud-based payment instruments supported? (#16)

In phase I, how do we support cloud-based payment instruments? These instruments require a user to:

1. Start on the merchant website
2. Click buy
3. Select the cloud-based instrument
4. Navigate to a 3rd party website to approve the transaction
5. Return to the merchant website with a token, approval code, or other proof that the transaction has been approved.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webpayments/issues/16