Where/how to support `fsspec` synchronous-only filesystems

[moradology]

In the process of trying to support changes for zarr v3 in kerchunk (issue here), I've implemented a little wrapper to make synchronous filesystem instances into AsyncFileSystems (pr here).

Unfortunately, in the process of applying necessary changes to get tests working in kerchunk, the simplest path was to modify the from_url method on RemoteStore. This becomes - roughly - this:

I get the sense that people may not want this kind of workaround in zarr-python itself but it's a bit difficult to see what the alternative is if we're going to support a code path using fsspec's url_to_fs utility. Thoughts?

[moradology]

Closing after conversation in #514