Changes to Create an external tax calculator

added the optional property data/attributes/type to the response with the 201 status

request property data/type was restricted to a list of enum values

removed the request property data/relationships/tax_categories

added the new optional request property data/attributes/external_includes

the type request property default value external_tax_calculators was removed

added the new external_tax_calculators enum value to the request property data/type

added the media type application/vnd.api+json for the response with the status 201