Implement an AddFields operation

[cmcarthur]

Is your feature request related to a problem or challenge?

I'm working on a Rust-based tool for managing data ingestion into Iceberg catalogs. We want this tool to manage schema evolution. Most frequently, these schema evolutions come in the form of adding new optional fields to existing tables: this allows for backwards compatibility in reading old files, as well as writing future files with additional optional fields.

Describe the solution you'd like

A public API with simple semantics that allows me to add a new column to an existing Iceberg table.

Willingness to contribute

I can contribute to this feature independently

[cmcarthur]

I have attempted a fix here: #1176 and verified that it works for our internal use case.