feat: Add Go helper functions for HttpResponse

[tomdaffurn]

builtin/external_module.go will have a couple of helper functions added:

// A helper for successful HttpResponse
//
func HttpSuccess[E any, B any](code int, body B) HttpResponse[B, E] {
  return HttpResponse[B, E]{Status: code, Body: ftl.Some(body)}
}

// A helper for HttpResponse with error
//
func HttpError[B any, E any](code int, e E) HttpResponse[B, E] {
  return HttpResponse[B, E]{Status: code, Error: ftl.Some(e)}
}

Fixes: #1632

[alecthomas]

Love it!