Returns the identity document types the onboarding API accepts for a given issuing
country, and which images each type needs. The compliance provider rejects an
associated_persons[].identity_files[] part that is not in required_parts for the
type and country (e.g. Malaysian NATIONAL_ID takes FRONT only), so read this
before collecting the documents. Returns an empty list when the reference data has
not been synced yet, or for a country we hold no document data for.