Add KvDiff - Represents a difference for a specific key, with properties

[zhangfengcdt]

1. KvDiff - Represents a difference for a specific key, with properties:
   - key: The key that changed (bytes)
   - operation: A DiffOperation object describing the change
2. DiffOperation - Represents the type of change, with properties:
   - operation_type: String ("Added", "Removed", or "Modified")
   - value: For Added/Removed operations (bytes or None)
   - old_value: For Modified operations (bytes or None)
   - new_value: For Modified operations (bytes or None)

Other Functions Not Yet Exposed:

I also identified these functions that haven't been exposed to Python yet:

• get_commits(key) - Get all commits that modified a specific key
• tree() and tree_mut() - Direct access to underlying ProllyTree
• git_repo() - Access to the underlying git repository