Add async/asyncio support

## Summary
`crate-python` is entirely synchronous. Every call to `Connection`, `Cursor.execute()`, `fetchall()`, and the `BLOB API` blocks the calling thread. This makes the driver unusable in modern async Python stacks like FastAPI route handlers, Django async views, Starlette/Litestar, asyncio-based ETL pipelines, and LangChain/LlamaIndex tool integrations that all expect await-able database calls.

This issue tracks the design and implementation of a first-class async interface for `crate-python`.

## Motivation
New support would increase use in:
- AI/ML backends: FastAPI services that call both a vector store and CrateDB in the same request handler
- Real-time dashboards: async WebSocket handlers that need concurrent DB queries
- High-throughput ingest: asyncio pipelines where thread-per-connection doesn't scale