Add support for pack download-sbom

[ryanmoran]

Summary

The pack CLI supports a new command, download-sbom, that allows users to download the launch SBOM from a container image. We should include this in occam so that we can leverage it in our testing processes.

PR: buildpacks/pack#1351

[fg-j]

It seems like pack build --sbom-output-dir dumps the build, launch, and cache SBOMs into the specified output directory. This means there will be a way to access launch SBOM for a build once completing #126. Is supporting this separate command still worth doing?

For an example, try building this java sample app with the instructions from the README and add the --sbom-output-dir flag.

[ryanmoran]

@fg-j If that's the case, then it makes sense to hold off on this until we have a situation that would require it.