feat(scan): implement size-based file scan task planning

[xbattlax]

Summary

Implement size-based file scan task planning for iceberg-rust, addressing issue #128.

Changes

• Add crates/iceberg/src/scan/bin_packing.rs with:

  • Greedy bin-packing algorithm with configurable lookback
  • BinPackingStream<S> for async streaming (memory efficient)
  • CombinedScanTask grouping for balanced parallel execution
  • Weight calculation considering data size and file open costs

• Update crates/iceberg/src/scan/context.rs:

  • File splitting based on split_offsets (Parquet row group boundaries)
  • Fallback to byte-range splitting when offsets unavailable
  • Optimized delete file handling (move ownership for last split)

• Update crates/iceberg/src/scan/mod.rs:

  • Add with_split_target_size(), with_split_open_file_cost(), with_split_lookback() builder methods
  • Add plan_tasks() method returning CombinedScanTaskStream

• Update crates/iceberg/src/scan/task.rs:

  • Add CombinedScanTask struct and CombinedScanTaskStream type

Notes

This matches the Java Iceberg implementation's TableScanUtil.planTasks() functionality:

• Large files are split into multiple tasks for parallel processing
• Small files are combined to reduce file open overhead
• Streaming implementation avoids collecting all tasks into memory

Closes #128