feat(iceberg): support metadata tables

[xxchan]

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What's changed and what's your intention?

support table functions iceberg_xxx, e.g.,

select * from iceberg_snapshots(iceberg_source);

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• My PR contains critical fixes that are necessary to be merged into the latest release.

Documentation

• My PR needs documentation updates.

Release note

[graphite-app]

The schema should be initialized using iceberg_metadata_table_schema(node.table_type) rather than Schema::new(vec![]) to properly reflect the metadata table's structure. The current empty schema means no columns would be available, which is incorrect.

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