nixos/networkd: add ipvtap kind to netdev and IPVTAP= to network

From `man systemd.netdev`: https://www.freedesktop.org/software/systemd/man/latest/systemd.netdev.html#Supported%20netdev%20kinds

From `man systemd.network`: https://www.freedesktop.org/software/systemd/man/latest/systemd.network.html#IPoIB=

Changed files
+2
nixos
modules
system
+2
nixos/modules/system/boot/networkd.nix
···
"ip6gretap"
"ipip"
"ipvlan"
"macvlan"
"macvtap"
"sit"
···
"VRF"
"VLAN"
"IPVLAN"
"MACVLAN"
"MACVTAP"
"VXLAN"
···
"ip6gretap"
"ipip"
"ipvlan"
+
"ipvtap"
"macvlan"
"macvtap"
"sit"
···
"VRF"
"VLAN"
"IPVLAN"
+
"IPVTAP"
"MACVLAN"
"MACVTAP"
"VXLAN"