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/auth-dovecot-users.conf.ext.j2

@@ -0,0 +1,12 @@
+# Dovecot local users authentication
+# Ansible managed: {{ ansible_managed }}
+
+passdb {
+  driver = passwd-file
+  args = scheme=SHA512-CRYPT username_format=%u /etc/dovecot/users
+}
+
+userdb {
+  driver = static
+  args = uid=vmail gid=vmail home=/var/vmail/%u
+}