Create a contact list, optionally importing rows in the same request

Rows carry phone_e164 plus optional name, language, time_zone (an IANA time zone, matched ignoring case and stored by its zone name; empty takes the number's zone when it has only one), external_reference, consent_basis and flat template variables (the attempt's whole call context: at most 32 keys and 4 KB, lists of at most 16 items; never secrets). Invalid rows are reported per row; duplicates within the list are skipped. At most 10,000 rows per request.