networkd: introduce netdev "Netdevsim" Driver

This "netdevsim" as implied by the name is a tool for network developers and is a simulator.
This simulated networking device is used for testing various networking APIs and at this time
is particularly focused on testing hardware offloading related interfaces.

src/network/netdev/netdevsim.h

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+/***
+  This file is part of systemd.
+
+  Copyright 2018 Susant Sahani
+***/
+
+typedef struct NetDevSim NetDevSim;
+
+#include "netdev/netdev.h"
+
+struct NetDevSim {
+        NetDev meta;
+};
+
+DEFINE_NETDEV_CAST(NETDEVSIM, NetDevSim);
+extern const NetDevVTable netdevsim_vtable;