Track remaining Iceberg v3 feature support (Roadmap for 0.5.0)

[manuzhang]

This is the parent tracking issue for work carried over from the 0.4.0 roadmap (#637) and targeted for the 0.5.0 release.

If you are interested in working on any of the items below, please open a dedicated sub-issue (or link the existing one) and label the PR with the 0.5.0 milestone so progress is easy to follow.

V3 related

1. variant type

• Schema-level support for variant (Support v3 variant data type #666) @zhjwpku @nssalian — depends on arrow-cpp variant support ([C++][Parquet] Add variant support arrow#45937); non-shredded variant first, shredded variant is a stretch goal

2. Column default values

• Apply write-default in Parquet and Avro writers (feat: apply write defaults in Parquet and Avro writers #942) @huan233usc
• Apply initial-default on the read path

3. Table-level encryption metadata (not yet planned)

• Parse and preserve encryption metadata

4. Puffin-based statistics (not yet planned)

• Metadata parsing
• Optional integration into scan planning

Table update and maintenance

• RewriteManifests (feat: add rewrite manifests #774) @zhjwpku
• CherryPickOperation (feat: add CherryPickOperation and wire SnapshotManager::Cherrypick #926) @shangxinli

If anything is missing or you have suggestions, feel free to comment or open additional sub-issues.

[wgtmac]

I think @shangxinli is the perfect expert to work on Table-level encryption metadata.