cosign tree support for OCI 1.1 Referrers

[ralphbean]

Description

In previous changes, support for the OCI Referrers API was added:

• Add support for new bundle specification for attesting/verifying OCI image attestations #3889 for attest and verify-attestation.
• Add COSIGN_OCI_EXPERIMENTAL, push .sig/.sbom using OCI 1.1+ digest tag #2684 for sign, attach, and download.

But, support was never added to cosign tree. If I sign, attach, or attest to a container image, those results don't appear in the results of cosign tree.

I was thinking that if we put this behind a cosign tree --experimental-oci11 flag, then it would make sense for cosign tree to display all artifacts found, both those located by the convention implemented by cosign triangulate and those located by the OCI 1.1 Referrers API.

Something like this:

❯ ./cosign tree --experimental-oci11 quay.io/rbean/tree-scsa:example
📦 Supply Chain Security Related artifacts for an image: quay.io/rbean/tree-scsa:example
└── 🔐 Signatures for an image tag: quay.io/rbean/tree-scsa:sha256-b3454025ce78fa93aba879b2824a06a59e2b8c5005b2a4286368433cd0f41cde.sig
   └── 🍒 sha256:ef4e39921eeb670ea453be070877d0fef225a13f97a9f9bf107d4d04d4c0dba6
└── 💾 Attestations for an image tag: quay.io/rbean/tree-scsa:sha256-b3454025ce78fa93aba879b2824a06a59e2b8c5005b2a4286368433cd0f41cde.att
   ├── 🍒 sha256:105da535ba7425024b03685048c33a458b3147315458fc1291479bae34f34997
   └── 🍒 sha256:98c6b2784ffe8a38cc4533d9a5d845cfe4ce72c2aa7e7f00a0682718d0259472
└── 🔗 application/vnd.dev.sigstore.bundle.v0.3+json artifacts via OCI referrer: quay.io/rbean/tree-scsa@sha256:01ca3469ae921632f7bd965cb34f9c10f07a723ea68223008871e86052bcfec8:
   └── 🍒 sha256:a336d1bfe93e60ae8a5cbbe546663517719d8f3ce1e628ffc0196c0cdf658314

(I already prepared a patch for this at #4205, but I was stopped by the pull request template which suggested that an issue should be on file with some time to discuss the right approach first.)

[lefosg]

I was about to make an issue about that. Indeed, if you sign an image using the oci 1.1 parameter, you cannot even verify the image. In my gitlab ci yaml file i have this line which signs a container image:

• COSIGN_EXPERIMENTAL=1 ./cosign sign --registry-referrers-mode=oci-1-1 "$CI_REGISTRY_IMAGE:1.0.5"
• ...
• COSIGN_EXPERIMENTAL=1 ./cosign attest -y --type spdxjson --predicate image.sbom.json "$CI_REGISTRY_IMAGE:1.0.5"
  Signature and sbom attestation appear in the tags. But cosign verify outputs Error: no signatures found. The sbom attestation is listed in the cosign tree command.