network-generator: pick up .netdev/.link/.network configuration via credentials

To me this is the last major basic functionality that couldn't be
configured via credentials: the network.

We do not invent any new format for this, but simply copy relevant creds
1:1 into /run/systemd/network/ to open up the full functionality of
networkd to VM hosts.

test/units/testsuite-74.network-generator.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# shellcheck disable=SC2016
+set -eux
+set -o pipefail
+
+at_exit() {
+    rm -f /run/credstore/network.network.50-testme
+    rm -f /run/systemd/system/systemd-network-generator.service.d/50-testme.conf
+}
+
+trap at_exit EXIT
+
+mkdir -p /run/credstore
+cat > /run/credstore/network.network.50-testme <<EOF
+[Match]
+Property=IDONTEXIST
+EOF
+
+systemctl edit systemd-network-generator.service --stdin --drop-in=50-testme.conf <<EOF
+[Service]
+LoadCredential=network.network.50-testme
+EOF
+
+systemctl restart systemd-network-generator
+
+test -f /run/systemd/network/50-testme.network