Add `enum` support for parameters

[mguentner]

I'd like to supply an enum when specifying a parameter like so:

option.Query("foobar", "Some query param.", param.Enum("a", "b", "c")),

Spec: https://swagger.io/docs/specification/v3_0/data-models/enums/

[dylanhitt]

This has been on my mind both for types, typed params, and in this style for a quite some time.

[dylanhitt]

Idea for making this work on request bodies as well: #222 (comment)