Add in-memory heap storage implementation

[r33drichards]

Summary

This PR adds an in-memory heap storage implementation for V8 heap snapshots, providing a fast, zero-configuration storage option ideal for development, testing, and ephemeral workloads.

Changes

New Implementation

• InMemoryHeapStorage: Thread-safe in-memory storage using HashMap with Arc<RwLock>
• Implements the HeapStorage trait for seamless integration
• Added to AnyHeapStorage enum as InMemory variant

CLI Updates

• Added --in-memory flag to enable in-memory storage
• Proper conflict handling with other storage options (--s3-bucket, --directory-path, --stateless)
• Enhanced logging to show which storage backend is being used

API Features

Core methods:

• put(name, data) - Store heap snapshot
• get(name) - Retrieve heap snapshot

Additional utilities:

• len() - Get number of stored items
• is_empty() - Check if storage is empty
• clear() - Remove all stored data
• contains_key(name) - Check if key exists
• remove(name) - Remove specific item
• keys() - Get all stored keys

Testing

• Comprehensive unit tests covering:
  • Basic put/get operations
  • Error handling for missing keys
  • Utility method functionality
  • Clone behavior (shared storage)
  • Integration with AnyHeapStorage

Documentation

• Added IN_MEMORY_STORAGE.md with:
  • Usage examples
  • Comparison with other storage backends
  • Limitations and best practices
  • Architecture details

Benefits

• Performance: Fastest storage option (no I/O overhead)
• Simplicity: No external dependencies or configuration
• Development: Perfect for local testing and iteration
• Thread-safe: Concurrent access with proper locking

Limitations

• Data is not persisted (lost on process termination)
• Memory bounded by available RAM
• Not suitable for production with persistent data requirements
• Single-process only (cannot be shared across instances)

Usage

# Run with in-memory storage
cargo run --bin server -- --in-memory

# With HTTP transport
cargo run --bin server -- --in-memory --http-port 8080

# With SSE transport
cargo run --bin server -- --in-memory --sse-port 8080

Testing

cargo test -p server heap_storage