nixos/networkd: allow IPv6PrivacyExtensions in networkd.conf

This change adds support for setting IPv6PrivacyExtensions= in
systemd.network.config.networkConfig.

https://www.freedesktop.org/software/systemd/man/latest/networkd.conf.html#IPv6PrivacyExtensions=

Added in systemd version 254.

Changed files
+2
nixos
modules
system
+2
nixos/modules/system/boot/networkd.nix
···
"ManageForeignRoutingPolicyRules"
"ManageForeignRoutes"
"RouteTable"
+
"IPv6PrivacyExtensions"
])
(assertValueOneOf "SpeedMeter" boolValues)
(assertInt "SpeedMeterIntervalSec")
(assertValueOneOf "ManageForeignRoutingPolicyRules" boolValues)
(assertValueOneOf "ManageForeignRoutes" boolValues)
+
(assertValueOneOf "IPv6PrivacyExtensions" (boolValues ++ ["prefer-public" "kernel"]))
];
sectionDHCPv4 = checkUnitConfig "DHCPv4" [