Customizable responses

[imalsogreg]

Currently the HTTP response is always assumed to be JSON, and we handle a response in a Raw API by converting that JSON into a dhall value with type outputType.

As discussed in #6, we should let the configuration determine how HTTP responses are handled. We can introduce a dhall type

type Response = { headers : List { key: Text, value: Text }, body : Text, status : Natural }

and allow adding a field to the Raw request API, e.g.:

  exampleAPI = {
    ...
    fromResponse = \(r : Response) -> dhallFromJSON r.body
  }