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

### 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

Start in extensions-contrib/druid-iceberg-extensions and compare its current behavior with the linked Iceberg format spec. Trace the existing read-parameter and type-mapping paths, then add coverage for snapshotId, ref, v3 unknown-to-null, and initial-default reads. Done means all four independent improvements are implemented and their tests pass.

Written by the indexing model from the issue text.