Wrap sync fs for xarray.to_zarr

[moradology]

This PR automatically wraps synchronous filesystems in the RemoteStore.from_url function. This is necessary unless we provide a force_async argument on fsspecs url_to_fs function.

I'm not sure adding more flags (even if we sugar them with default values) is the best move. Another option might be to implement a url_to_asyncfs method that simply wraps url_to_fs and wraps any synchronous filesystems that come out automatically but that, too, would require changes here.

This PR is in service of downstream kerchunk compatibility: fsspec/kerchunk#516

It depends on: fsspec/filesystem_spec#1755