Import Contacts
PUT {{baseUrl}}/marketing/contacts/imports
This endpoint returns a URL and HTTP headers which can be used to PUT a file of contacts.
field_mappings is a respective list of field_definition IDs to map the uploaded CSV columns to. 
eg: field_mappings = [null, "w1", "_rf1"] means to skip Col[0], map Col[1] => CustomField w1, map Col[2] => ReservedField _rf1 .  See CustomFields Get All Field Definitions
 endpoint to fetch the IDs.
This call requires a second API call where you use the URL and HTTP header given to upload you