Add missing ORC iceberg.required attribute

[ebyhr]

Rationale for this change

Iceberg spec expects iceberg.required attribute in addition to iceberg.id:

The column IDs must be stored in ORC type attributes using the key iceberg.id, and iceberg.required to store "true" if the Iceberg column is required, otherwise it will be optional.

https://iceberg.apache.org/spec/#orc

Fixes #2526

Are these changes tested?

Yes

Are there any user-facing changes?

[geruh]

Hey @ebyhr, this is great! Just have the tiny comment below.

[Fokko]

This looks great! Thanks @ebyhr for picking this up, and thanks @geruh for the review 🙌