Add Iceberg output format and Postgres ingest flow

[kraftaa]

Summary

This PR adds Apache Iceberg support to rustream and includes an ingest path for loading S3/local data into Postgres.
It is rebased on top of reliability improvements from main (idempotent incremental checkpoints + composite cursor paging).

What’s included

1. Iceberg write support

• new Iceberg writer pipeline and table write flow
• load-or-create table behavior
• commit via Iceberg transaction path

2. Catalog integration

• Iceberg catalog wiring and config support
• warehouse/catalog initialization for Iceberg mode

3. Sync integration

• sync path supports format = iceberg
• Parquet path remains available
• incremental cursor logic from main preserved

4. Ingest subcommand

• adds S3/local -> Postgres ingest command path

5. Docs/config updates

• README updates for Iceberg + ingest usage
• config examples updated for new format options

Important behavior note

• Incremental correctness remains based on composite cursor (incremental_column + incremental_tiebreaker_column) and checkpointed progress state.
• For Iceberg mode, progress is checkpointed after successful Iceberg commit.

Files to review first

• src/iceberg.rs
• src/catalog.rs
• src/sync.rs
• src/main.rs
• src/config.rs
• README.md

Validation performed

• cargo fmt
• cargo test

Risk / follow-up

• Add integration tests for Iceberg commit + resume semantics (especially duplicate watermark scenarios).
• Validate catalog compatibility across target environments (warehouse path/backends).