fix: revert Apache datafusion-iceberg adoption (#742) - #749

## Summary

Reland
[#742](#742)
as a narrow dependency/predicate-conversion refactor. Add Apache
`iceberg-datafusion` pinned to `20327db`, use its public
`read::convert_filters_to_predicate`, and remove DFD's 931-line copied
converter and tests. DFD's table providers, data source, work-unit feed,
codec, worker reader, statistics, and task assignment are unchanged.
This does not depend on the open Apache eager-planning PR.


[#749](#749)
reverted #742 because its Apache pin pulled iceberg-rust `665c64e`,
which lacked working `FileScanTask` serialization:
`executes_with_estimated_scan_tasks` panicked and hung. The pinned
Apache revision (`20327db`, current Apache `main` when tested) includes
#21 and uses iceberg-rust `5b82ab9`, which retains `FileScanTask`
serialization. Both DFD's direct `iceberg` and `iceberg-storage-opendal`
dependencies now use that same revision. The existing distributed
regression test, including worker task transport, passes. No DFD
protocol or work-unit encoding code changes; mixed-version rolling
upgrades were not tested.

The iceberg-rust pin changes types exposed through DFD's public Iceberg
APIs; `docs/upgrade/5.0.0.md` tells users with direct iceberg-rust
dependencies to align their revision or use DFD's `iceberg` re-export.

## Validation

- `cargo check -p datafusion-distributed-iceberg` — pass.
- `cargo test --locked -p datafusion-distributed-iceberg --features
integration --test desired_task_count
tests::executes_with_estimated_scan_tasks -- --exact` — pass (the #749
reproducer).
- `cargo test --locked -p datafusion-distributed-iceberg --features
integration` — pass, including predicate pushdown, codecs, statistics,
and distributed execution.
- `cargo test --locked -p datafusion-distributed --lib` — pass (319
passed, 1 ignored).
- `cargo clippy --locked -p datafusion-distributed-iceberg --features
integration --all-targets -- -D warnings` — pass.
- `cargo fmt --all -- --check` and `git diff --check` — pass.

## AI disclosure

Developed with AI assistance; changes were reviewed and validated
locally before opening this draft.