- add two classes conforming to BUYPaymentSessionProvider for internal use
- modify the API to only return payment tokens wrapped by protocol
- merge two `completePayment` methods into a single method

Also update to support new JSON serialization and dirty tracking.

Standardize category names and unify to have one category per class. Move files to "Additions" directory.

This adds a attributes to checkout. These objects are returned as an array of dictionaries, but expected as dictionaries when posting the checkout, so we're required to serialize the objects correctly. API now also returns the `note` on a checkout, so I have added a test for that.