Add flake for NixOS servers support

[jim3692]

This a NixOS wrapper around the installation script for easier integration on NixOS servers.

It runs Docker Compose as a systemd service and exposes the CLI configuration options as nix options, ex:

{ ... }: {
    piped = {
        frontend = "piped.kavin.rocks";
        backend = "pipedapi.kavin.rocks";
        proxy = "pipedproxy.kavin.rocks";
        reverseproxy = "caddy";
    };
}

It also exposes some additional options:

{ ... }: {
    piped = {
        httpPort = 80; # Default
        httpsPort = 443; # Default
        dataDir = "/var/lib/piped"; # Default
    };
}