Add V3 type `unknown`

[Fokko]

Feature Request / Improvement

According to the spec: https://iceberg.apache.org/spec/?column-projection#primitive-types

[kaushiksrini]

hey @Fokko, I'd like to work on this if it's available

[kevinjqliu]

sure @kaushiksrini feel free to create a PR and reference this issue :)