dnscrypt-proxy: enable additional plugins

Adding ldns builds additional plugins for rewriting DNS queries,
such as blocking and forwarding.

For example, to use a custom domain blocklist, you can do
```nix
let
  myBlockListFile = writeText "blocked-domains" ''
    *.example.com
  '';
in
{
  services.dnscrypt-proxy.extraArgs = [
    "-X libdcplugin_example_ldns_blocking.so,--domains=${myBlockListFile}"
  ];
}
```