Export a collection of contacts to a CSV file. Supply fields to specify which contact columns appear in the CSV (including custom attribute keys). The action runs asynchronously — poll the returned record with GET /api/v2/contacts/export_actions/{id}. Once complete, file_url contains the download URL for the generated CSV.
Exactly one target selection must be supplied: stable_id, stored_filter_id (alias filter_id), target_ids, or target_all. Returns 422 if none is provided or if the supplied value is invalid.