Iceberg proto representations of tech spec

### Feature Request / Improvement

Hi team,
quick thought on the tech spec: would it make sense to add an Iceberg proto module? We recently did a POC and found that this really solves the pain points of building a client-server app on top of the Iceberg REST catalog. It gives us a solid data model for client-server communication, and it makes managing and switching between Iceberg spec versions much easier. Let me know what you think!

### Query engine

None

### Willingness to contribute

- [x] I can contribute this improvement/feature independently
- [x] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

The issue names no files, tests, or entry points. Start by reviewing the Iceberg REST catalog and the tech-spec version requirements; clarify the proto module's supported representations and client-server scope with the community. Done means an agreed module design that handles the intended spec versions.

Written by the indexing model from the issue text.