Adds support for contacts (Cards) marked as `external_resource` which
prevents modifications from happening. No way to import external
contacts is added yet.
CardId is the "Primary Key" of the contact type, Card. Meli-created
contacts had UUIDs for their CardId. In order to import external
contacts and ensure their primary key is the same each time, CardId is
made into an enum to add hashing as a choice.
Manos Pitsidianakis