✨(domains) get domain expected config for DNS

Call dimail to get DNS configuration values
to make an external domain work and save it in our db.
Add values to serializer for displaying.

src/backend/mailbox_manager/models.py

@@ -36,6 +36,12 @@ class MailDomain(BaseModel):
         verbose_name=_("last check details"),
         help_text=_("A JSON object containing the last health check details"),
     )
+    expected_config = models.JSONField(
+        null=True,
+        blank=True,
+        verbose_name=_("expected config"),
+        help_text=_("A JSON object containing the expected config"),
+    )
 
     class Meta:
         db_table = "people_mail_domain"