async cloud I/O support

[kajbaf]

It would be very cool to add support for asynchronous operations.

Most similar libraries like smart_open and even the standard gcs and s3 libraries doesn't support async.

The only libraries that I've seen implemented async mode are fsspec's set of libraries and gcloud-aio-storage. But none of them are as feature rich as pyfilesystem. The former is focused on providing API for Dask and Pandas, and the latter is not convenient at all.

I know that you have already rejected issue https://github.com/PyFilesystem/pyfilesystem2/issues/163; but with the rise of async http libraries like aiohttp and httpx, this would be much easier to achieve.

It would be great if you define base async functionality in specifications, until other collaborators volunteer to help you on this.