Adds compute-artifact-description action

[dmikusa]

Summary

This action will go through and compute a list of all the dependencies used by the buildpack. It is recursive, so if you point it to a composite buildpack, it'll pull all of the dependencies for every buildpack included in that composite buildpack and give one list, sorted and with duplicates removed.

Use Cases

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).