Preload oras://ghcr.io SIF images for better UX

[maouw]

Preloads oras://ghcr.io SIF images for better UX -- shows download progress.

This should not be necessary as of Apptainer v1.2.3 (2023-09-14):

The apptainer push/pull commands now show a progress bar for the oras protocol like there was for docker and library protocols.

But klone has apptainer 1.1.5 installed. Ideally, we should be able to remove this when klone upgrades the apptainer version.

The implementation uses curl to query the ghcr API in roughly the same way apptainer does it in internal/pkg/client/oras/oras.go. It uses python3 to parse the JSON responses and downloads the blob for the image directly to $APPTAINER_CACHEDIR/cache/oras, which is where apptainer looks for a cached image from oras://ghcr.io (This apptainer cache behavior might change in the future.).

If anything goes wrong, the preload will fail and hyakvnc will carry on.