Run the Noise handlers under a new struct so we can access the noiseConn from the handlers

In TS2021 the MachineKey can be obtained from noiseConn.Peer() - contrary to what I thought before,
where I assumed MachineKey was dropped in TS2021.

By having a ts2021App and hanging from there the TS2021 handlers, we can fetch again the MachineKey.

protocol_legacy.go

@@ -56,5 +56,5 @@ func (h *Headscale) RegistrationHandler(
 		return
 	}
 
-	h.handleRegisterCommon(writer, req, registerRequest, machineKey)
+	h.handleRegisterCommon(writer, req, registerRequest, machineKey, false)
 }