Credential payment api

[ghost]

why not?

[googlebot]

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[ghost]

https://cla.developers.google.com/ to signed.

[googlebot]

CLAs look good, thanks!

[keanulee]

Not interested in having these changes in the main master branch, but will keep the credential-payment-api branch around so users can see the changes required (see my opinion at #91 (comment)).

[ghost]

I will implement it in a real-life example of SHOP. :.-(