Add Save Payment Method

[jshawl]

This PR implements the Save Payment Method (during purchase) integration as described by the documentation.

I followed the standard-integration example as closely as possible with a few notable differences:

• generateAccessToken no longer calls the REST API directly
• 👆 authenticate is used for obtaining an id token and an access token in the same request
• I'm not sure there's a link back to the documentation anywhere in the code or readme, but that would be a nice enhancement!