Support working with OCI manifests directly

[craciunoiuc]

Feature request summary

The OCI package needs to be revisited in order to enable pushes/pulls with Manifests directly.

Right now, only Indexes are supported, which in turn forces you to also specify a platform and/or architecture in order to pinpoint to a single Manifest inside an Index.

kraft pkg export -u --plat qemu "unikraft.org/hugo@sha256:4f2e2fc2aa92a754ede22a6ba36a6b24fc3c8543558534458365c183ba20c77e" --as oci

With this fix this example would become:

kraft pkg export -u "unikraft.org/hugo@sha256:c77a965082c061dece821c484c379b8c24455d06fafd5a550db055b4b9e820f0" --as oci

But right now it errors out with:

could not pull all components: could not retrieve remote index: GET https://unikraft.org/v2/hugo/manifests/sha256:c77a965082c061dece821c484c379b8c24455d06fafd5a550db055b4b9e820f0: MANIFEST_UNKNOWN: OCI manifest found, but accept header does not support OCI manifests

The directory handler has some support, but is incomplete, and bug-ridden.

Describe alternatives

No alternative currently to working directly with indexes

Related architectures

None

Related platforms

None

Additional context

No response