realm: Add maximum file size upload restriction.

This commit adds a restriction to the maximum file size
that can be uploaded to a realm based on its plan_type.

zerver/lib/event_schema.py

@@ -1075,6 +1075,7 @@ plan_type_data = DictType(
     required_keys=[
         ("plan_type", int),
         ("upload_quota_mib", OptionalType(int)),
+        ("max_file_upload_size_mib", int),
     ],
 )