Add Enter DFU mode admin message

meshtastic/admin.proto

@@ -231,6 +231,12 @@ message AdminMessage {
      */
     NodeRemoteHardwarePinsResponse get_node_remote_hardware_pins_response = 20;
 
+    /*
+     * Enter (serial) DFU mode
+     * Only implemented on NRF52 currently
+     */
+    bool enter_dfu_mode_request = 21;
+
     /*
      * Set the owner for this node
      */
@@ -350,4 +356,4 @@ message NodeRemoteHardwarePinsResponse {
    * Nodes and their respective remote hardware GPIO pins
    */
   repeated NodeRemoteHardwarePin node_remote_hardware_pins = 1;
-}
+}