Add RNS Tunnel App Portnum

- Added RNS tunnel app to Port 76
  - Encoding is handled by RNS_Over_Meshtastic app

meshtastic/portnums.proto

@@ -206,6 +206,12 @@ enum PortNum {
    */
   POWERSTRESS_APP = 74;
 
+  /*
+   * Reticulum Network Stack Tunnel App
+   * ENCODING: Fragmented RNS Packet. Handled by Meshtastic RNS interface
+   */
+  RETICULUM_TUNNEL_APP = 76;
+
   /*
    * Private applications should use portnums >= 256.
    * To simplify initial development and testing you can use "PRIVATE_APP"