Add self link and a create-order rel link to OpportunityCollection returned by the product_router

[parksjr]

See comment on pr #80 here: #80 (comment)

Also extend this for OrderCollection.
background: we are changing the get_orders endpoint to return an OrderCollection (like OpportunityCollection) instead of list[Order]
see comment on pr #80 here: #80 (comment)

[jkeifer]

I found #47 does address the OpportunityCollection link. I am not sure it can be rebased effectively at this point, but the code there might serve as an effective start at implementing that link.

[philvarner]

I'm going to add the create-order link, but it's not conventional for a search endpoint like /opportunities to return a self link unless there's actually an identifiable persistent resource created