Changes to Create a new form

added the non-success response with the status 429

added the optional property lists/items/subscriber_description to the response with the 201 status

added the optional property lists/items/public_description to the response with the 201 status

added the optional property lists/items/subscribed_contacts_count to the response with the 201 status

added the optional property lists/items/unsubscribed_contacts_count to the response with the 201 status

endpoint added