feat(network): add fleet endpoint registry

lib/network-endpoints.nix

@@ -0,0 +1,42 @@
+{ lib }:
+let
+  endpointOptions = {
+    address = lib.mkOption {
+      type = lib.types.str;
+      example = "10.23.20.126";
+      description = "Expected IP address assigned outside NixOS for this network endpoint.";
+    };
+  };
+
+  endpointType = lib.types.submodule {
+    options = endpointOptions;
+  };
+
+  endpointFields = endpoint: {
+    inherit (endpoint) address;
+  };
+
+  collectFleetEndpoints =
+    hosts:
+    lib.concatMapAttrs (
+      hostName: host:
+      let
+        cfg = host.config.modules.network;
+        hostEndpoint = lib.optionalAttrs (cfg.endpoint.address != null) {
+          ${hostName} = endpointFields cfg.endpoint;
+        };
+        guestEndpoints = lib.mapAttrs' (
+          endpointName: endpoint: lib.nameValuePair "${hostName}-${endpointName}" (endpointFields endpoint)
+        ) cfg.endpoints;
+      in
+      hostEndpoint // guestEndpoints
+    ) hosts;
+in
+{
+  inherit
+    collectFleetEndpoints
+    endpointFields
+    endpointOptions
+    endpointType
+    ;
+}