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.

Start by reviewing the async AsyncClient and the surrounding asyncio-based stack, then read the linked anyio and Trio documentation to understand the backend boundary. Done means users can choose either asyncio or Trio without maintaining separate stacks, while existing asyncio-using code remains compatible.