Add create table as select support for iceberg.

[liurenjie1024]

Fixes #13521 .

Description

Add create table as select support for iceberg.

Checklists

• This PR has added documentation for new or modified features or behaviors.
• This PR has added new tests or modified existing tests to cover new code paths.
  (Please explain in the PR description how the new code paths are tested, such as names of the new/existing tests that cover them.)
• Performance testing has been performed and its results are added in the PR description. Or, an issue has been filed with a link in the PR description.

[Copilot]

Pull Request Overview

This PR adds support for Iceberg CREATE TABLE AS SELECT (CTAS) operations on GPU by extending the existing Iceberg provider infrastructure to handle AtomicCreateTableAsSelectExec operations.

Key changes:

• Extended IcebergProvider interface with CTAS support methods
• Added GPU tagging and conversion logic for CTAS operations
• Implemented comprehensive test coverage for various CTAS scenarios

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
ExternalSource.scala	Added CTAS support to GPU tagging and conversion logic
IcebergProvider.scala	Extended interface with CTAS-specific methods
iceberg_ctas_test.py	Comprehensive test suite covering CTAS scenarios and fallback cases
GpuSparkWrite.scala	Added CTAS-specific GPU tagging method with validation logic
package.scala	Added catalog support utility function
IcebergProviderImpl.scala	Implemented CTAS support methods with GPU conversion

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[res-life]

LGTM

[liurenjie1024]

build