Added the ability to merge orders into an additional order via the API

In API V8, the method MarkOrderAsAdditional has been added, which allows you to designate the order current as an additional order to the order parent. The property GroupOrderId of the current order is set equal to the property Id of the parent order, or to null if parent == null.

The current order cannot be null, the parent can be null if you need to cancel the merging of orders into an additional order. The current order must be open, the parent can be either an open or closed order. Both the current and parent orders cannot be delivery orders.