Make parent on OrderItem expandable

[remi-stripe]

Make parent on OrderItem expandable. This helps fix part of #1515

This is a tricky one though. First because it's a breaking change as we original chose Parent and not ParentId as the name. Second, because parent is only expandable when it refers to a SKU and not otherwise. In stripe-go this is implemented here. In stripe-java, we avoided the issue by simply assuming that the expanded object is HasId.

Here, I used Sku but the alternative is likely to build an Interface just for it as we do for other polymorphic properties. Wanted to get your opinion first though.

r? @ob-stripe
cc @stripe/api-libraries

[ob-stripe]

That's fine, parent can only be a Sku. (Initially it could be either a Sku or a Discount, but that never worked properly because discounts don't have IDs, which is a prerequisite for expansion. This was fixed a few months back.)

[remi-stripe]

@ob-stripe Can you make that part of your next major release? Avoids a major version just for this.