Expose Iceberg data file row positions

[osipovartem]

Summary

• add an opt-in DataFusionTable configuration field for the existing Iceberg file row-position virtual column
• map requested projections to the physical scan schema by field name so internal delete sequence columns cannot shift requested columns
• keep the provider-side hidden field nullable for INSERT compatibility while scan values remain non-null
• extend the hidden-column scan test to validate row positions

Validation

• cargo +1.95.0 test --offline -p datafusion_iceberg test_datafusion_table_insert_with_data_file_path --lib
• cargo +1.95.0 clippy --offline -p datafusion_iceberg --all-targets
• cargo +1.95.0 fmt --all
• git diff --check