channel_folder: Add API to create a channel folder.

This commit also includes code to include channel_folders
data in register response.

Fixes part of #31972.

zerver/migrations/0707_realmauditlog_modified_channel_folder.py

@@ -0,0 +1,20 @@
+# Generated by Django 5.1.8 on 2025-05-05 13:27
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("zerver", "0706_channelfolder"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="realmauditlog",
+            name="modified_channel_folder",
+            field=models.ForeignKey(
+                null=True, on_delete=django.db.models.deletion.CASCADE, to="zerver.channelfolder"
+            ),
+        ),
+    ]