feat: add support for local dovecot users via passwd-file

This introduces functionality to manage local Dovecot users utilizing a
static `vmail` system user and a flat password file.

Key changes:
- Added `dovecot_users` configuration list to defaults.
- Implemented creation of `vmail` user and group (uid/gid 5000).
- Added logic to generate a random security token using `pwgen` to prefix
  user passwords.
- Created `auth-dovecot-users.conf.ext` and `dovecot-users.j2` templates
  to handle `passwd-file` authentication.
- Updated `10-auth.conf` to include the new local users configuration.
- Updated README with usage instructions and token details.

templates/10-auth.conf.j2

@@ -4,4 +4,5 @@
 disable_plaintext_auth = {{ 'yes' if dovecot_ssl == 'required' else 'no' }}
 auth_mechanisms = {{ dovecot_auth_mechanisms }}
 
+!include auth-dovecot-users.conf.ext
 !include auth-system.conf.ext