Support #/components/responses

[Tomboyo]

Lily must read the #/components/responses section of the OAS and generate re-usable response objects from it.

We are already able to generate responses from anonymous definitions in operations, which created sealed interfaces (MyResponse permits MyResponse200, MyResponse404, ...). However, that code doesn't account for $refs when creating the interfaces (that work is deferred to this ticket). Because responses are generated as a sealed interface, Lily needs to make $referenced responses from /components/responses members of each API response that uses them. So, for example, if GetFoo and BetBar operations rely on a shared Generic500Response schema, then Generic500Response's class must implement GetFooResponse and GetBarResponse.