fix: Add support for unsigned Arrow datatypes in schema conversion

[GeetKrishna]

Which issue does this PR close?

• Closes bug: ArrowSchemaConverter can't handle unsigned datatypes from arrow #675
• Fixed ArrowSchemaConverter to handle unsigned Arrow datatypes (UInt8, UInt16, UInt32, UInt64) through byte-to-byte conversion with metadata preservation.

What changes are included in this PR?

Bug Fixes

• Fixed crash when ArrowSchemaConverter encounters unsigned datatypes
• Resolved "Unsupported Arrow data type" errors for UInt8/16/32/64

Features

• Added round-trip conversion support for all unsigned Arrow types
• Implemented metadata-based type preservation using field documentation
• Added byte-to-byte conversion: UInt8/16/32 → Int32, UInt64 → Int64

Code Changes

• Added constants: UNSIGNED_TYPE_PREFIX, ARROW_FIELD_UNSIGNED_KEY
• Added helper functions: get_unsigned_type_name(), restore_unsigned_type()
• Enhanced ArrowSchemaConverter: Support for unsigned types in primitive() method
• Enhanced ToArrowSchemaConverter: Restoration logic in field() method
• Added comprehensive tests: test_unsigned_type_conversion() for all unsigned variants

Files Modified

• crates/iceberg/src/arrow/schema.rs

Impact

• ✅ No breaking changes - existing functionality preserved
• ✅ No data loss - all unsigned values preserved through conversion
• ✅ Backward compatible with existing Iceberg schemas
• ✅ Minimal performance overhead only for unsigned types

Are these changes tested?

• All existing schema tests pass
• New comprehensive test covers UInt8, UInt16, UInt32, UInt64 round-trip conversion