nixos/networkmanager: add extraConfig

nixos/modules/services/networking/networkmanager.nix

@@ -38,6 +38,8 @@ let
 
     [device]
     wifi.scan-rand-mac-address=${if cfg.wifi.scanRandMacAddress then "yes" else "no"}
+
+    ${cfg.extraConfig}
   '';
 
   /*
@@ -120,6 +122,14 @@ in {
         '';
       };
 
+      extraConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Configuration appended to the generated NetworkManager.conf.
+        '';
+      };
+
       unmanaged = mkOption {
         type = types.listOf types.string;
         default = [];