Start an async contact import

Import a batch of contacts (up to 5,000 per request) in the background. The call returns immediately with an import job you poll for progress and per-row results — new contacts are created and existing ones (matched by email) are skipped, not changed. Track it with Retrieve a contact import, or see all your imports via List contact imports. To add a single contact synchronously, use Create a contact. Requires the contacts:write capability.