Motivation for this change

Some firewalls performs deep packet inspection to block VPN traffic, such as the Greaf Firewall (GFW) of the Chinese government. Therefore it is required to obfuscate the traffic which can be done using the v2ray software. This PR introduces the v2ray-plugin for the shadowsocks VPN and makes the shadowsocks service for NixOS aware of plugins.

Personally, I'm using this in production for myself and for a friend in Beijing for a couple of months without problems. This is what my configuration roughly looks like:

{
  networking.firewall = {
    enable = true;
    allowedTCPPorts = [ 8488 ];
    allowedUDPPorts = [ 8488 ];
  };
  services.shadowsocks = {
    enable = true;
    encryptionMethod = "chacha20-ietf-poly1305";
    passwordFile = "/path/to/shadowsocks/key";
    localAddress = [ "0.0.0.0" ];
    port = 8488;
    mode = "tcp_and_udp";
    plugin = "${pkgs.v2ray-plugin}/bin/v2ray-plugin";
    pluginOpts = "server;host=henrimenke.com";
  };
}

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.