This adds a `withPlugins` function to Caddy package.
```nix
services.caddy = {
enable = true;
package = pkgs.caddy.withPlugins {
plugins = [ "github.com/caddy-dns/powerdns@v1.0.1" ];
hash = "sha256-F/jqR4iEsklJFycTjSaW8B/V3iTGqqGOzwYBUXxRKrc=";
};
};
```