Support SPC as an interaction mode for outgoing payments

[adrianhopebailie]

In the use case where an online merchant is processing a payment where the customer has provided their payment pointer, the merchant, if they detect that the browser is capable of Secure Payment Confirmation, should indicate this is an option for interaction.

The interaction would follow the example in the spec:
https://www.w3.org/TR/secure-payment-confirmation/#sctn-sample-authentication

The flow would look like this:

1. Customer provides PP
2. Merchant starts grant negotiation for quote and outgoing payment, indicates SPC as an interaction option
3. AS returns list of credential IDs that can be used for SPC and RP identifier
4. Merchant initiates SPC and customer consents to the transaction
5. Merchant continues grant negotiation and provides SPC response
6. AS verifies SPC response and checks that details in SPC match outgoing payment details.
7. AS grants access token to create outgoing payment