Add Heartbeat message to clientonly.proto

meshtastic/clientonly.proto

@@ -40,3 +40,11 @@ message DeviceProfile {
    */
   optional LocalModuleConfig module_config = 5;
 }
+
+/* 
+ * A heartbeat message is sent by a node to indicate that it is still alive.
+ * This is currently only needed to keep serial connections alive.
+ */
+message Heartbeat {
+  
+}