[CH] Support reading data from the iceberg with CH backend

### Description

[CH] Support reading data from the iceberg with CH backend
- [x] basic iceberg scan transformer
- [x] read from the iceberg with the copy-on-write mode
- [ ] read from the iceberg with the merge-on-read mode

The issue identifies the CH backend, an existing basic Iceberg scan transformer, and copy-on-write support; begin by tracing those paths. Investigate the existing Iceberg read behavior and determine how merge-on-read support should be verified, then confirm that CH-backed scans handle merge-on-read data.

Written by the indexing model from the issue text.