Opportunity properties should be optional on `Product`

[gadomski]

Not all products will support opportunities, so we shouldn't always have to provide opportunity_properties:

pystapi/stapi-fastapi/src/stapi_fastapi/models/product.py

Line 36 in 0f36066

opportunity_properties: type[OpportunityProperties],

[jkeifer]

The order model is based on a flawed and erroneous understanding of mine as to the relationship between orders and opportunities. It needs to be reconsidered from the ground up and much better specified in the spec, hence stapi-spec/stapi-spec#267.

More to the point here, the idea that an order has one and only one opportunity is just wrong. Orders could relate in some way to 0 or more opportunities in various ways. Simply making the opportunity properties optional does not resolve this flaw.