use product constraint models to define models used in endpoints

[c-wygoda]

goal: let FastAPI document all possible input/output models of endpoints based on the defined products and their constraints instead of only showing generic models.

[jkeifer]

I believe this is addressed by #80, as much as is possible given only some endpoints are product-specific. I'm going to go ahead and close this issue; if the work in #80 didn't address these concerns effectively please feel free to reopen, or open new issues for specific examples.