async support

[davetapley]

Following on from:

• Support async I/O duckdb#3560 (comment)

[dhirschfeld]

It should probably be a feature request issue?

[evertlammerts]

I’ve been working on making query execution truly async in core for a while now, both in the engine and in the API. It’s getting there, but I can’t promise that it’ll be available in our Python package in v2.0 immediately. We first need a couple of other less visible changes to land.

[dhirschfeld]

I'm interested in trio so would appreciate if the async support could be built on top of anyio rather than asyncio. That will allow it to run on top of either event loop and has the benefit of requiring correct-by-construction structured concurrency.