Added restrictions on unsupported fields in payloads

Every endpoint of the Import API now accepts only supported fields in input payloads. Any Import API POST request with an unsupported field input will fail with an InvalidFieldError.

Previously, the Import API's POST endpoints did not impose such restrictions and ignored unsupported extra fields without reporting an error.

This change will help keep things consistent in the Import API because the same HTTP POST request is used for creating and updating a resource.