Allow describing the Set-Cookie header

[handrews]

From @bali182 with substantial discussion contributions from @flaksp at #90 (comment).

We are already looking at modeling headers better, but Set-Cookie is a bit unusual among headers, so I'm splitting this discussion out on its own. See also:

• How to handle response headers? #22
• Modeling HTTP fields (headers/trailers) #108

[handrews]

We pretty much answered this for OAS 3.2. It might be worth revisiting as we model headers in general, but we've got an approach for the special-caseness this was talking about.