Allow using S3 and GCS in file explorer, but without using FUSE drivers at all

[williamstein]

Again in the JupyterLab meeting right now and they are talking about a "custom file browser" that uses a google cloud storage credentials (NOT FUSE) and lets you browse files there. They are NOT mounted in any way on the underlying computer. Nonetheless you can browse directories, create notebooks/files, open them, etc.

We could support this as well. It would be nontrivial but could be very useful for some use cases we have discussed.