Import Contacts

**This endpoint allows a CSV upload containing up to one million contacts or 5GB of data, whichever is smaller. At least one identifier is required for a successful import.** Imports take place asynchronously: the endpoint returns a URL (`upload_uri`) and HTTP headers (`upload_headers`) which can subsequently be used to `PUT` a file of contacts to be imported into our system. Uploaded CSV files may also be [gzip-compressed](https://en.wikipedia.org/wiki/Gzip). In either case, you must incl...