Support content type negotiation

[bessgeor]

OpenAPI spec allows to define different types for different content types.
It would be nice to generate something like that for this case:

type SomeMethodBodyInput =
  | SomeMethodBodyJson of SomeMethodJsonBodyInput
  | SomeMethodBodyFormUrlEncoded of SomeMethodFormUrlEncodedBodyInput
//...
match ctx.Request.Headers.ContentType with
| "text/json" ->
  let! json = ctx.bindJsonAsync<SomeMethodJsonBodyInput>()
  json
  //...
  |> SomeMethodBodyJson
  |> Ok
//...
| unsupported -> UnsupportedBodyContentType unsupported |> BodyBindingError |> Error