nixos/v2ray: change the type of config field

Currently, the type of `config` is `types.attrsOf types.unspecified`. It results
that the merging of list doesn't perform as expected.

For example, I declare two part of config in two files:

`a.nix`:
```
config = {
  routing = {
    rules = [
      {
        type = "field";
        outboundTag = "direct";
        domain = [ "geosite:cn" ];
      }
    ];
  };
};
```

`b.nix`:
```
config = {
  routing = {
    rules = [
      {
        type = "field";
        outboundTag = "direct";
        ip = [
          "geoip:cn"
          "geoip:private"
        ];
      }
    ];
  };
};
```

After merging, only the content of `b.nix` left, the content of `a.nix` is
removed.

This commit give the right type of config, which makes the merging of config
acts as expected.

nixos/modules/services/networking/v2ray.nix

@@ -2,7 +2,9 @@
 
 with lib;
 
-{
+let
+  json = pkgs.formats.json { };
+in {
   options = {
 
     services.v2ray = {
@@ -39,7 +41,7 @@ with lib;
       };
 
       config = mkOption {
-        type = types.nullOr (types.attrsOf types.unspecified);
+        type = types.nullOr json.type;
         default = null;
         example = {
           inbounds = [{