Muuntaja – OpenAPI integration

[opqdonut]

Currently both openapi and muuntaja features need to be given lists of supported content-types. If we've specified content-types for a route, muuntaja should use those.

Related to #84

• [x] Level 1: get list of supported content types from muuntaja for openapi docs (mostly just for the :content {:default ..} case)
• [ ] Level 2: have muuntaja only accept content types explicitly listed in the :request :content and :response :content maps (:default will still work of course)

[opqdonut]

Level 1 was in #663