networkd: add extraConfig to all units

networkd options are always correct or up to date. This option allows to by
pass type checking. It is also easier to write because examples can be just copy
and paste from manpages.

nixos/modules/system/boot/networkd.nix

@@ -165,6 +165,11 @@ let
       '';
     };
 
+    extraConfig = mkOption {
+      default = "";
+      type = types.lines;
+      description = "Extra configuration append to unit";
+    };
   };
 
   linkOptions = commonNetworkOptions // {
@@ -515,6 +520,8 @@ let
         ''
           [Link]
           ${attrsToSection def.linkConfig}
+
+          ${def.extraConfig}
         '';
     };
 
@@ -565,6 +572,7 @@ let
             ${attrsToSection def.bondConfig}
 
           ''}
+          ${def.extraConfig}
         '';
     };
 
@@ -603,6 +611,7 @@ let
             ${attrsToSection x.routeConfig}
 
           '')}
+          ${def.extraConfig}
         '';
     };