Iceberg input source: roadmap of v2 polish + v3-readiness improvements

[Shekharrajak]

Description

Small, independent improvements to
extensions-contrib/druid-iceberg-extensions that complete v2 spec coverage
and add basic v3 readiness:

1. Expose snapshotId parameter for time-travel by snapshot ID
2. Expose ref parameter to read from a branch or tag
3. Map Iceberg v3 unknown type to null
4. Test column initial-default values on read

Reference

• Iceberg spec: https://github.com/apache/iceberg/blob/main/format/spec.md