feat: being able to automatically pull and record attached SBOMS  #244

A common use-case for people building and signing container images is to use cosign. Cosign allows not only to sign such container images but also in-toto attestations and SBOMS.

An operator, using Chainloop, might want to leverage this container image metadata and inject it as pieces of evidence.

Today this is possible by manually downloading the sbom (i.e oras pull) and then adding it to the attestation chainloop att add ....

We should simplify this process by automatically downloading, extracting and attaching such sboms.

One way we could do it is by extending the contract format

From

schemaVersion: v1

materials:
  - type: CONTAINER_IMAGE
    name: image
    output: true

  - type: SBOM_CYCLONEDX_JSON
    name: sbom

to

schemaVersion: v1

materials:
  - type: CONTAINER_IMAGE
    name: image
    output: true
    sboms:
      - type: SBOM_CYCLONEDX_JSON
        name: sbom

note how now operators can indicate that the container image to be added must "contain" an sbom. This new contract will indicate Chainloop CLI to find and download the sbom and attach it as sbom material type.

Note that this SBOMs might some as in-toto predicates too so we need to make sure we support not only raw spdx/cyclonedx artifacts but also in-toto specs