[iceberg] Tag columns with "partition key" in DESCRIBE and SHOW COLUMNS output

[imjalpreet]

Description

After the change:

Motivation and Context

Fixes #22638

Contributor checklist

• [x] Please make sure your submission complies with our development, formatting, commit message, and attribution guidelines.
• [x] PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• [x] Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• [x] If release notes are required, they follow the release notes guidelines.
• [ ] Adequate tests were added if applicable.
• [ ] CI passed.

Release Notes

== NO RELEASE NOTE ==