feat(dovecot): add support for configuring imap_capability

This introduces the `dovecot_imap_capability` variable to allow customization of advertised IMAP capabilities.

- Add `dovecot_imap_capability` to `defaults/main.yml` (defaulting to empty).
- Update `templates/dovecot.conf.j2` to conditionally include the `protocol imap` block if the capability string is provided.
- This enables operators to add or remove specific IMAP extensions (e.g., disabling `LITERAL+` or `NOTIFY`) for client compatibility.

templates/dovecot.conf.j2

@@ -2,6 +2,12 @@
 # Ansible managed: {{ ansible_managed }}
 
 protocols = {{ dovecot_protocols }}
+{% if dovecot_imap_capability | default('') | length > 0 %}
+
+protocol imap {
+  imap_capability = {{ dovecot_imap_capability }}
+}
+{% endif %}
 
 # Dictionary of configuration files
 !include conf.d/*.conf