Add PortNum 77 for Cayenne Low Power Payload (#709)

meshtastic/portnums.proto

@@ -217,6 +217,13 @@ enum PortNum {
    */
   RETICULUM_TUNNEL_APP = 76;
 
+  /*
+   * App for transporting Cayenne Low Power Payload, popular for LoRaWAN sensor nodes. Offers ability to send
+   * arbitrary telemetry over meshtastic that is not covered by telemetry.proto
+   * ENCODING: CayenneLLP
+   */
+  CAYENNE_APP = 77;
+
   /*
    * Private applications should use portnums >= 256.
    * To simplify initial development and testing you can use "PRIVATE_APP"