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
	};
}

[Bnyro]

Suggested change

	httpmode= mkOption {
	description = "HTTP mode to access your instance (either http or https)";
	type = types.str;
	};

was added recently, thus that buildscript wouldn't work anymore

[Bnyro]

Generally looks good, if you're willing to update it to be compatible/working with the latest Piped version I think that should be good to merge.