Proposal: Cherry-pick pxf-iceberg feature from `GreengageDB/pxf` to Apache Cloudberry PXF

## Background

Currently, Cloudberry offers integration with Apache Iceberg through the FDW (Foreign Data Wrapper) solution, and related PRs are being actively pushed forward. However, users who are already familiar with PXF may prefer a PXF-native approach for reading/writing Iceberg tables.

The [GreengageDB/pxf](https://github.com/GreengageDB/pxf) project has implemented an `pxf-iceberg` extension that enables PXF to intergate Apache Iceberg tables. Since GreengageDB/pxf is licensed under Apache 2.0, and [apache/cloudberry-pxf](https://github.com/apache/cloudberry-pxf) is also licensed under Apache 2.0, there is no license conflict or incompatibility in cherry-picking this feature.

## Proposal

Cherry-pick the pxf-iceberg feature from [GreengageDB/pxf](https://github.com/GreengageDB/pxf) into [apache/cloudberry-pxf](https://github.com/apache/cloudberry-pxf), so that users can choose between the FDW approach and the PXF approach when integrating Apache Iceberg with Cloudberry.

## Benefits

- Lower barrier for PXF users — Users already familiar with PXF can leverage their existing knowledge and workflows to query Iceberg tables.
- Broader ecosystem support — Offering both FDW and PXF pathways gives Cloudberry users more flexibility.
- License compatibility — Both repositories use Apache 2.0, so there are no legal obstacles.
- Proven implementation — The feature already exists and works in GreengageDB/pxf, reducing the development effort.

## Considerations / Open Questions

- What is the scope of code changes required for the cherry-pick?
- Are there any dependencies that need to be ported alongside the core feature?

## Related

- Existing FDW-based Iceberg integration PRs [GreengageDB/pxf — pxf-iceberg](https://github.com/GreengageDB/pxf)

Start by reviewing the existing pxf-iceberg implementation in GreengageDB/pxf and comparing it with apache/cloudberry-pxf. Identify the code and dependencies that must be ported, then confirm that the feature integrates with Cloudberry PXF and supports the proposed Iceberg workflow.

Written by the indexing model from the issue text.