Later in this patch series, `sl:driver` will be added as a special
service level created automatically by the system. It needs special
handling in `DESC SCHEMA ...` to ensure that during backup restore:
1. CREATE SERVICE LEVEL does not fail if `sl:driver` already exists
2. If `sl:driver` exists, its configuration is fully restored (emit
ALTER SERVICE LEVEL).
3. If `sl:driver` was removed, the information is retained (emit
DROP SERVICE LEVEL instead of CREATE/ALTER).
Refs: scylladb/scylladb#24411
5cb4577800