feat(spec): read partition and sort fields with multi-argument transforms (source-ids)

[moomindani]

Which issue does this PR close?

• Closes Reading v3 metadata with multi-argument transform fields (source-ids) fails #2801. Part of EPIC: v3 Support Tracking #2411.

What changes are included in this PR?

The v3 spec serializes partition fields and sort fields whose transform takes multiple arguments with a source-ids list instead of the single source-id. Deserializing such metadata failed with missing field source-id, so a v3 table containing any multi-argument transform could not be read at all.

This adds read tolerance, mirroring PyIceberg (apache/iceberg-python#3630):

• PartitionField and SortField accept source-ids: a single-element list is normalized onto source-id; a multi-element list is kept in the new source_ids field and the transform maps to Transform::Unknown — multi-argument transforms cannot be evaluated, and the spec requires v3 readers to read such tables ignoring them. source_id holds the first id so existing consumers keep working.
• Serialization follows the spec's writer rule: single-argument transforms write only source-id, multi-argument transforms write only source-ids, so the field round-trips.
• PartitionSpec::is_compatible_with also compares source_ids, so two multi-arg fields sharing the first id no longer compare as identical.
• SortField's Display renders all ids for multi-argument fields.
• PartitionSpec::partition_type works unchanged: Transform::Unknown produces string per the spec.

Note: this adds a public source_ids field to PartitionField and SortField (will be flagged by cargo-public-api). Known limitation shared with #2790: Transform::Unknown does not retain the original transform name, so round-tripping writes "unknown"; name preservation stays tracked in #2789.

Are these changes tested?

Yes — new unit tests in spec/partition.rs and spec/sort.rs: multi-arg deserialization and round-trip, single-element normalization, empty source-ids rejection, missing-transform rejection, and partition_type over a multi-arg field. cargo test -p iceberg --lib (1399 tests), cargo clippy -p iceberg --lib --tests, and cargo check --workspace --tests pass locally.

This pull request and its description were written by Claude Fable 5.