Add `systemd.network.networks.*.dhcpServerStaticLeaseConfig` to allow
for configuring static DHCP leases through the `[DHCPServerStaticLease]`
section. See systemd.network(5) of systemd 249 for details.
Also adds the NixOS test `systemd-networkd-dhcpserver-static-lease` to
test the assignment of static leases.
