Provide a way to attach virtual folders

[achimnol]

Maybe we could offer to set an environment variable (e.g., BACKEND_VFOLDER) to specify a vfolder name that is always mounted into every kernel the integration launches.

It would be more natural to use a magic command in Jupyter notebooks, but the mount must be done at the time of kernel creation -- so it's far more better to support the Jupyter service served via HTTP tunneling directly from kernel containers later.