modules: services: add postgresql

Enable the service itself in other modules when needed, but pin the
package in a single place.

modules/services/postgresql.nix

@@ -0,0 +1,18 @@
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.my.services.postgresql;
+in
+{
+  options.my.services.postgresql = with lib; {
+    enable = my.mkDisableOption "postgres configuration";
+  };
+
+  config = lib.mkMerge [
+    # Let other services enable postgres when they need it
+    (lib.mkIf cfg.enable {
+      services.postgresql = {
+        package = pkgs.postgresql_12;
+      };
+    })
+  ];
+}