Add category information to the checkout confirmation object

Drupal 7

# Add a checkboxes list on the global Affirm settings form where you identify all of your product category related taxonomy vocabularies.
# On checkout complete, look for $line_item->data['context']['entity'] where the entity_type is node.
# If you find one, load it, load its bundle information, and see which taxonomy vocabularies its term reference fields reference.
# For any that match vocabularies enabled in the settings form, include their referenced term names (not IDs) in a list in the checkout object.

- API function just gets the enabled vocabularies
- API function to identify term reference fields that match those vocabularies

Drupal 8

# Add the same checkboxes list to the settings form.
# From the product variation, load its product, and do the same as above.