🗃️(domains) add "secret" field to domains

add a "secret" field to domain model. This secret will be used as
password in mail provisioning API.

src/backend/mailbox_manager/migrations/0011_maildomain_secret.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.0.6 on 2024-07-01 16:22
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mailbox_manager', '0010_alter_mailbox_first_name_alter_mailbox_last_name'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='maildomain',
+            name='secret',
+            field=models.CharField(blank=True, max_length=255, null=True, verbose_name='secret'),
+        ),
+    ]