feat: add iterators

[willothy]

[!IMPORTANT] Add Iter struct and implement Iterator trait for ShardMap to support iteration over key-value pairs.

• Behavior:
  • Add Iter struct in src/iter.rs to iterate over ShardMap key-value pairs.
  • Implement Iterator trait for Iter to support iteration.
  • Add iter() method to ShardMap in src/shard_map.rs to return an Iter instance.
• Tests:
  • Add test_iter and test_iter_empty in src/iter.rs to verify iterator functionality.
• Misc:
  • Add read_sync() and write_sync() methods to Shard in src/shard.rs for synchronous access.
  • Remove path filters from .github/workflows/rust.yml for pull requests.

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