nixos/networking: add foo-over-udp endpoint support

allows configuration of foo-over-udp decapsulation endpoints. sadly networkd
seems to lack the features necessary to support local and peer address
configuration, so those are only supported when using scripted configuration.

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -47,6 +47,9 @@ in
     } ] ++ flip mapAttrsToList cfg.bridges (n: { rstp, ... }: {
       assertion = !rstp;
       message = "networking.bridges.${n}.rstp is not supported by networkd.";
+    }) ++ flip mapAttrsToList cfg.fooOverUDP (n: { local, ... }: {
+      assertion = local == null;
+      message = "networking.fooOverUDP.${n}.local is not supported by networkd.";
     });
 
     networking.dhcpcd.enable = mkDefault false;
@@ -194,6 +197,23 @@ in
           macvlan = [ name ];
         } ]);
       })))
+      (mkMerge (flip mapAttrsToList cfg.fooOverUDP (name: fou: {
+        netdevs."40-${name}" = {
+          netdevConfig = {
+            Name = name;
+            Kind = "fou";
+          };
+          # unfortunately networkd cannot encode dependencies of netdevs on addresses/routes,
+          # so we cannot specify Local=, Peer=, PeerPort=. this looks like a missing feature
+          # in networkd.
+          fooOverUDPConfig = {
+            Port = fou.port;
+            Encapsulation = if fou.protocol != null then "FooOverUDP" else "GenericUDPEncapsulation";
+          } // (optionalAttrs (fou.protocol != null) {
+            Protocol = fou.protocol;
+          });
+        };
+      })))
       (mkMerge (flip mapAttrsToList cfg.sits (name: sit: {
         netdevs."40-${name}" = {
           netdevConfig = {