exim: add extraRouters option

krebs/3modules/exim-smarthost.nix

@@ -39,6 +39,11 @@ let
       default = [];
     };
 
+    extraRouters = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+    };
+
     internet-aliases = mkOption {
       type = types.listOf (types.submodule ({
         options = {
@@ -254,6 +259,8 @@ let
           transport = home_maildir
           cannot_route_message = Unknown user
 
+        ${lib.optionalString (cfg.extraRouters != null) cfg.extraRouters}
+
         begin transports
 
         retiolum_smtp: