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

首先映射现有的 Connection、Cursor.execute()、fetchall() 和 BLOB API 入口点，以了解同步层面。检查这些调用在当前驱动程序中的使用方式，并定义第一类异步接口必须覆盖的内容。完成的标准是，异步 Python 调用方可以使用可 await 的等价接口，而不会阻塞调用线程。

由索引模型根据 Issue 内容生成。