Use `anyio` for async code, so that users can choose either `asyncio` or `trio` backends

👋 Over at @anthropics we're enthusiastic users of both Slack and async Python - but instead of asyncio we use structured concurrency with Trio.

This makes using slack-bolt's AyncClient pretty awkward, because now we have multiple async backends running, can't use our standard helper functions, and things generally start going wrong. Building a copy of the whole stack for each backend isn't feasible (thus also sans-io designs), and hence this request to support using either asyncio or trio as the backend.

Using anyio makes this pretty easy - just ask e.g. FastAPI! - and it should be fully compatible with existing asyncio-using code. I realize it's not a particularly small task, but Trio is downloaded about 200k times per day and good support for that makes a big difference.

Beginne mit der Überprüfung des asynchronen AsyncClient und des umgebenden asyncio-basierten Stacks. Lies anschließend die verlinkte Dokumentation zu anyio und Trio, um die Backend-Grenze zu verstehen. Das Ziel ist erreicht, wenn Benutzer entweder asyncio oder Trio verwenden können, ohne separate Stacks zu pflegen, während bestehender Code, der asyncio verwendet, kompatibel bleibt.