Support Swagger "examples"

[Limess]

Add support for documenting curated swagger examples, see https://swagger.io/docs/specification/2-0/adding-examples/

The current schema driven data is excellent, but it'd be great to also support examples with sensible defaults and actual data, rather than just types.

[ikitommi]

You can currently add any swagger data using :swagger key, either to route-data or to models. Example here: https://github.com/metosin/reitit/blob/master/examples/http-swagger/src/example/server.clj#L32-L37 & https://cljdoc.org/d/metosin/reitit/0.5.15/doc/ring/swagger-support