Factory supervisor for macula_request children.

Unlike macula_response_sup (owned internally by the SDK, one per advertised procedure), this is meant to be embedded in the *caller's* own supervision tree: the application decides when to start a new outbound request and when to cancel one via supervisor:terminate_child/2 (or macula_request:cancel/1 on the child pid directly). Children are temporary — a finished or crashed request is never restarted.