realm: Add uuid and associated secret columns.

Thisi and the following commit follow the approach used in
3e2ad84bbe.

First migration requires a server restart - after that any new realms
will be created with the columns set.

The following migrations are in the next commit:

Second migration does a backfill for older realms and can run in the
background while the server is operating normally.

Third migration enforces null=False now that all realms have the columns
set.

zerver/migrations/0479_realm_uuid_realm_uuid_owner_secret.py

@@ -0,0 +1,22 @@
+# Generated by Django 4.2.5 on 2023-10-06 09:21
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0478_realm_enable_guest_user_indicator"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="realm",
+            name="uuid",
+            field=models.UUIDField(null=True, unique=True),
+        ),
+        migrations.AddField(
+            model_name="realm",
+            name="uuid_owner_secret",
+            field=models.TextField(null=True),
+        ),
+    ]