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.

defaults/main.yml

@@ -58,4 +58,11 @@ dovecot_auth_mechanisms: "plain login"
 
 # Postfix integration
 dovecot_postfix_sasl_enable: true
-dovecot_postfix_lmtp_enable: true
+dovecot_postfix_lmtp_enable: true
+
+# Local Dovecot Users
+# Example:
+# dovecot_users:
+#   - name: "service1"
+#     pass: "secret123"
+dovecot_users: []