add: stalwart package, nixos module

ripped from nixpkgs, maintaining stalwart myself for a bit

pkgs/default.nix

@@ -26,4 +26,5 @@
   yubilock = pkgs.callPackage ./linux/yubilock/default.nix { };
   ani-cli-rs = pkgs.callPackage ./linux/ani-cli-rs.nix { };
   opengoal = pkgs.callPackage ./linux/opengoal.nix { };
+  stalwart = pkgs.callPackage ./common/stalwart/stalwart.nix { };
 }