nixos/networkd: add [IPVLAN] and [IPVTAP] configuration options to systemd.netdev files

[IPVLAN](https://www.freedesktop.org/software/systemd/man/latest/systemd.netdev.html#%5BIPVLAN%5D%20Section%20Options)
[IPVTAP](https://www.freedesktop.org/software/systemd/man/latest/systemd.netdev.html#%5BIPVTAP%5D%20Section%20Options)

nixos/lib/systemd-network-units.nix

···
     '' + optionalString (def.vlanConfig != { }) ''
       [VLAN]
       ${attrsToSection def.vlanConfig}
+    '' + optionalString (def.ipvlanConfig != { }) ''
+      [IPVLAN]
+      ${attrsToSection def.ipvlanConfig}
+    '' + optionalString (def.ipvtapConfig != { }) ''
+      [IPVTAP]
+      ${attrsToSection def.ipvtapConfig}
     '' + optionalString (def.macvlanConfig != { }) ''
       [MACVLAN]
       ${attrsToSection def.macvlanConfig}

nixos/modules/system/boot/networkd.nix

···
         (assertValueOneOf "PacketInfo" boolValues)
         (assertValueOneOf "VNetHeader" boolValues)
       ];
+
+      # See https://www.freedesktop.org/software/systemd/man/latest/systemd.netdev.html#%5BIPVTAP%5D%20Section%20Options
+      ipVlanVtapChecks = [
+        (assertOnlyFields [
+          "Mode"
+          "Flags"
+        ])
+        (assertValueOneOf "Mode" ["L2" "L3" "L3S" ])
+        (assertValueOneOf "Flags" ["private" "vepa" "bridge" ])
+      ];
     in {
 
       sectionNetdev = checkUnitConfig "Netdev" [
···
         (assertValueOneOf "LooseBinding" boolValues)
         (assertValueOneOf "ReorderHeader" boolValues)
       ];
+
+      sectionIPVLAN = checkUnitConfig "IPVLAN" ipVlanVtapChecks;
+
+      sectionIPVTAP = checkUnitConfig "IPVTAP" ipVlanVtapChecks;
 
       sectionMACVLAN = checkUnitConfig "MACVLAN" [
         (assertOnlyFields [
···
         Each attribute in this set specifies an option in the
         `[VLAN]` section of the unit.  See
         {manpage}`systemd.netdev(5)` for details.
+      '';
+    };
+
+    ipvlanConfig = mkOption {
+      default = {};
+      example = { Mode = "L2"; Flags = "private"; };
+      type = types.addCheck (types.attrsOf unitOption) check.netdev.sectionIPVLAN;
+      description = lib.mdDoc ''
+        Each attribute in this set specifies an option in the `[IPVLAN]` section of the unit.
+        See {manpage}`systemd.netdev(5)` for details.
+      '';
+    };
+
+    ipvtapConfig = mkOption {
+      default = {};
+      example = { Mode = "L3"; Flags = "vepa"; };
+      type = types.addCheck (types.attrsOf unitOption) check.netdev.sectionIPVTAP;
+      description = lib.mdDoc ''
+        Each attribute in this set specifies an option in the `[IPVTAP]` section of the unit.
+        See {manpage}`systemd.netdev(5)` for details.
       '';
     };