Apache Iceberg Rust version

None

Describe the bug

According to the iceberg spec documentation the field "identifier-field-ids" is an optional field.
See: https://iceberg.apache.org/spec/#schemas

But the type definition for it in rust is not marked as an Option.
See: https://github.com/apache/iceberg-rust/blob/main/crates/iceberg/src/spec/schema/mod.rs#L61C27-L61C40

Should be an easy change to comply with the official spec documentation, I can prepare a merge request for this.

To Reproduce

Expected behavior

Should match official iceberg spec

Willingness to contribute

I can contribute a fix for this bug independently