feat: Add comprehensive snapshot expiration functionality for table maintenance

[ForeverAngry]

Summary

This PR adds comprehensive(ish) (i think) snapshot expiration functionality to iceberg-rust, enabling automatic cleanup of old snapshots and associated files according to configurable retention policies.

Features Added

Core Functionality

• Snapshot Expiration API: New ExpireSnapshots builder with fluent configuration
• Multiple Retention Strategies:
  • Time-based expiration (expire_older_than)
  • Count-based retention (retain_last)
  • Combined criteria with precedence rules
• Safety Guarantees:
  • Never expire current snapshots
  • Preserve snapshots referenced by branches/tags
  • Atomic operations following Iceberg's commit model
• File Cleanup: Optional cleanup of orphaned manifest and data files
• Dry Run Mode: Preview functionality without making changes

API Example

// Expire snapshots older than 7 days, keeping at least 5 snapshots
let result = table.expire_snapshots()
    .expire_older_than(chrono::Utc::now().timestamp_millis() - 7 * 24 * 60 * 60 * 1000)
    .retain_last(5)
    .clean_orphan_files(true)
    .execute()
    .await?;

println!("Expired {} snapshots", result.expired_snapshot_ids.len());

Files Added/Modified

New Files

• iceberg-rust/src/table/maintenance/ - New maintenance module
• iceberg-rust/src/table/maintenance/expire_snapshots.rs - Core implementation (1,007 lines)
• iceberg-rust/examples/expire_snapshots.rs - Usage examples
• iceberg-rust/tests/snapshot_expiration.rs - Comprehensive integration tests

Modified Files

• iceberg-rust/src/table/mod.rs - Added expire_snapshots() method to Table
• iceberg-rust/src/lib.rs - Exposed maintenance module

Testing

Comprehensive Test Coverage

• 9 Unit Tests: Core functionality, edge cases, validation logic
• 6 Integration Tests: End-to-end scenarios with real table metadata
• All Tests Passing: ✅ 15/15 tests pass (I included my supporting tests as well)

Test Categories

• Time-based expiration logic
• Count-based retention logic
• Current snapshot protection
• Reference-aware cleanup
• Combined criteria precedence
• Empty metadata handling
• File cleanup identification

Implementation Details

Documentation

• Module-level documentation explaining concepts
• Some example (demo) code demonstrating common usage patterns
• Updated the main readme.md with the new features.

Compatibility

• ✅ All existing tests pass
• ✅ No breaking changes to existing APIs
• ✅ Compatible with latest upstream changes (merged 62 commits)

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This implementation provides a solid foundation for table maintenance operations in iceberg-rust, following Icebergs specification, and similar to that of the implementation from pyiceberg (that I also worked on 😮‍💨 ) in Rust!

[ForeverAngry]

@JanKaul let me know what you think :)