feat(topology): add kea network extractor

topology/nixos/extractors/kea.nix

@@ -0,0 +1,34 @@
+{
+  config,
+  lib,
+  ...
+}: let
+  inherit
+    (lib)
+    const
+    filter
+    genAttrs
+    hasInfix
+    mkEnableOption
+    mkIf
+    ;
+in {
+  options.topology.extractors.kea.enable = mkEnableOption "topology kea extractor" // {default = true;};
+
+  config = let
+    interfaces = filter (x: !hasInfix "*" x) config.services.kea.dhcp4.settings.interfaces-config.interfaces or [];
+    headOrNull = xs:
+      if xs == []
+      then null
+      else builtins.head xs;
+    subnet = headOrNull (map (x: x.subnet) (config.services.kea.dhcp4.settings.subnet4 or []));
+    netName = "${config.topology.self.id}-kea";
+  in
+    mkIf (config.topology.extractors.kea.enable && config.services.kea.dhcp4.enable && interfaces != [] && subnet != null) {
+      topology.networks.${netName} = {
+        cidrv4 = subnet;
+      };
+
+      topology.self.interfaces = genAttrs interfaces (const {network = netName;});
+    };
+}