✨(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/migrations/0021_maildomain_expected_config.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.1.6 on 2025-02-14 17:47
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mailbox_manager', '0020_alter_mailbox_options_alter_maildomain_options_and_more'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='maildomain',
+            name='expected_config',
+            field=models.JSONField(blank=True, help_text='A JSON object containing the expected config', null=True, verbose_name='expected config'),
+        ),
+    ]