Introduce stateVersion concept

With upcoming changes to the dovecot home and maildirectories we need to
introduce a way to nudge users to inform themselves about manual
migration steps they might need to carry out.

The idea here is to allow us to safely make breaking changes and notify
the user of required migration steps at eval time, so they can make the
necessary changes in time.

default.nix

@@ -25,6 +25,22 @@ in
   options.mailserver = {
     enable = mkEnableOption "nixos-mailserver";
 
+    stateVersion = mkOption {
+      type = types.nullOr types.ints.positive;
+      default = null;
+      description = ''
+        Tracking stateful version changes as an incrementing number.
+
+        When a new release comes out we may require manual migration steps to
+        be completed, before the new version can be put into production.
+
+        If your `stateVersion` is too low one or multiple assertions may
+        trigger to give you instructions on what migrations steps are required
+        to continue. Increase the `stateVersion` as instructed by the assertion
+        message.
+      '';
+    };
+
     openFirewall = mkOption {
       type = types.bool;
       default = true;