Add a Response object

This object will abstract the filter response. For now, it only allow
not to respond. This will change in a future version.

opensmtpd/src/lib.rs

@@ -26,6 +26,10 @@ macro_rules! handlers {
     };
 }
 
+pub enum Response {
+    None,
+}
+
 struct SessionHandler {
     entry_rx: mpsc::Receiver<Entry>,
     event_handlers: Vec<EventHandler>,