Add async Python client

[SK1PPR]

Summary

• Adds AsyncVortexDB as a separate async client class
• Uses grpc.aio for non-blocking gRPC calls
• Mirrors the existing sync client API
• Adds async context manager support
• Adds async client and async connection tests
• Adds async usage documentation and example
• Closes Add asynchronous support to the Python client #68

Testing

• python -m compileall vortexdb tests
• uv run --extra dev pytest -q