BulkOperation.bulkId is required when method is POST

[azmeuk]

RFC7644 indicates that BulkOperation.bulkId is required when method is POST:

  bulkId  The transient identifier of a newly created resource,
     unique within a bulk request and created by the client.  The
     bulkId serves as a surrogate resource id enabling clients to
     uniquely identify newly created resources in the response and
     cross-reference new resources in and across operations within a
     bulk request.  REQUIRED when "method" is "POST".

There should be a validator for this