Scan SBOM attestation

[knqyf263]

Trivy generates SBOM and cosign uploads it in an in-toto attestation format to OCI registry.
https://aquasecurity.github.io/trivy/dev/docs/attestation/sbom/

Also, Trivy can scan SBOM (currently CycloneDX only) for vulnerabilities.
https://aquasecurity.github.io/trivy/dev/docs/sbom/cyclonedx/

The next step is consuming the SBOM attestation and scanning it for vulnerabilities. There are some use cases as far as I know.

1. Fetch attestation of a container image from OCI registry, extract SBOM and scan it for vulnerabilities
2. cosign verify-attestation returns verified attestation, so Trivy takes it as input and scans it for vulnerabilities
3. Take a local file including attestation and scan it

We may want to add a new subcommand for attestation like trivy attestation that takes attestation as input. Then, the commands for each above could be as below.

1. $ trivy image --attest knqyf263/myapp:latest
2. $ cosign verify-attestation --key cosign.pub knqyf263/myapp:latest | trivy attestation -
3. $ trivy attestation ./vuln.json

Feedback is welcome.

[knqyf263]

@otms61 Could you take it?

[otms61]

Alright. I will try.