Generic Storage Backend Support for VersionedKvStore

[zhangfengcdt]

This PR enhances the VersionedKvStore to support all storage backends (InMemory, File, RocksDB, Git) while maintaining
backward compatibility and keeping Git as the default storage backend.

Key Changes

Core Implementation

• Modified VersionedKvStore to be generic over storage type <const N: usize, S: NodeStorage>
• Added StorageBackend enum in types.rs to track which storage backend is being used
• Created type aliases for convenient usage:
  • GitVersionedKvStore (default, maintains backward compatibility)
  • InMemoryVersionedKvStore
  • FileVersionedKvStore
  • RocksDBVersionedKvStore (when rocksdb_storage feature enabled)

Storage-Specific Initialization

• Added backend-specific init() and open() methods for each storage type
• Maintained Git-specific functionality like reload_tree_from_head() only for Git storage
• Updated all imports and usages across the codebase to use appropriate type aliases

Usage Examples

  // Git storage (default, backward compatible)
  let store = GitVersionedKvStore::<32>::init(&path)?;

  // In-memory storage
  let store = InMemoryVersionedKvStore::<32>::init(&path)?;

  // File storage
  let store = FileVersionedKvStore::<32>::init(&path)?;

  // RocksDB storage (with feature flag)
  let store = RocksDBVersionedKvStore::<32>::init(&path)?;

Benefits

1. Flexibility - Users can choose the most appropriate storage backend for their use case
2. Performance - Access to optimized storage backends like RocksDB for high-performance scenarios
3. Development - InMemory storage for testing, File storage for simple persistence
4. Production - Git storage for version control integration, RocksDB for high-throughput applications
5. Backward Compatibility - Existing code works without changes

This enhancement significantly expands the versatility of ProllyTree while maintaining its core versioning capabilities
across all storage backends.