Asynchronously supplement attributes before import.
This means prompting the user for things like labels and the like. The needed attributes will have been passed to supplement_prep.
This method prompts the user and fills in the attributes.
| this |
the interaction |
| builder |
supplemented attributes |
| cancellable |
optional cancellable object |
| callback |
called when the operation completes |
| user_data |
data to be passed to the callback |