Codegen custom helpers for builtins

[alecthomas]

It is very tedious dealing with builtin.HttpRequest/HttpResponse. Let's update the external module template with some custom helpers for that module. Something like:

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

func HttpError[B any, E any](code int, e E) HttpResponse[B, E] {
  return HttpResponse{Status: code, Error: ftl.Some(e)}
}

Which would replace:

return builtin.HttpResponse[MyBody, MyError]{Status: 200, Body: ftl.Some(MyBody{Field: "something"})}

With:

return builtin.HttpSuccess[MyError](200, MyBody{Field: "something"})

etc.