Changes to Create an external payment

added the new required request property data/relationships/order/data

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

removed the request property data/relationships/order/id

removed the request property data/relationships/order/type

the type request property default value external_payments was removed

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

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