Support non-https registries

[thesayyn]

Currently, diffoci only seems to support https registries, which prevents usage staging local registries to perform diffing against a remote image.

[AkihiroSuda]

As a workaround you may use ctr or nerdctl to pull the image, and use diffoci --backend=containerd --containerd-socket=SOCKET --containerd-namespace=NAMESPACE

https://github.com/reproducible-containers/diffoci?tab=readme-ov-file#accessing-containerd-images

[gregseth]

Could be nice to allow self-signed certificate as well.