postgres: Ensure creation of database users

This is just a proof of concept that I plan to integrate into NixOS
containers running specific users. The ensureDBOwnership part would no
longer be needed since each database would receive its own container
and consequently user.

modules/postgres.nix

@@ -1,7 +1,23 @@
 { pkgs, ... }:
 
 {
-  services.postgresql.enable = true;
+  services.postgresql = {
+    enable = true;
+
+    ensureUsers = [
+      {
+        name = "user";
+      }
+      {
+        name = "cooldbname";
+        ensureDBOwnership = true;
+      }
+    ];
+
+    ensureDatabases = [
+      "cooldbname"
+    ];
+  };
 
   environment.systemPackages = with pkgs; [
     pgcli