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

- switch from channelID to appID
- adjust API endpoints for products and collections
- adjust mocked responses

Also improve error description.

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.

cleanup error handling for Apple Pay
Use dispatch_groups to ensure the shop and checkout are completed before continuing with an Apple Pay checkout