Add support to pull LVH images directly from OCI repositories

[joestringer]

This PR adds support to directly pull VM images in the CLI so that the user
doesn't need to do this if they just want to reuse existing images that were
already built for the Cilium project.

The first command added is a direct replacement for the image pull script.
It pulls an image from the specified OCI repository and decompresses it
for local usage. Example usage:

$ lvh images pull quay.io/lvh-images/kind:bpf-next-main

The second command leverages the same underlying support in order to directly
run VM images from the OCI image. This removes the need for users to manually
pull it. Example usage:

$ lvh run --image quay.io/lvh-images/kind:bpf-next-main

[kkourt]

Thanks!

One thought I had was to add a manifest file in the images that describe what the OCI image contains (e.g., rootfs images, kernels, etc.). But until we have that, auto-detection is great.

[joestringer]

Yeah the autodetect is not entirely ideal since I did notice some docker images have multiple VM images inside. Autodetect is arbitrarily picking one. But then it also extracts every image, so if you know what you're doing then you can use the image pull and then subsequently explore the _data/images directory and pick the one you want. A manifest file could help with this, and we could potentially add some additional flag or syntax to pick between them if there are multiple.

Another potential area for expansion is to add a similar functionality for kernels (--kernel), which seem to be often published in separate docker images. I briefly looked into this, and it seemed like the kernel docker images published for LVH are not using bzImage so I wasn't quite sure how to extract + override the image settings to allow a specific kernel docker image to override the base LVH image kernel. Anyway, this is what I needed for now, I won't make any promises about digging further into that idea.

I fixed up the flag as you suggested, will merge. Thanks for the review!