feat: Code / Artifact Signing

[pmengelbert]

Make code / artifact signing possible via the spec. For example, windows artifacts can be signed. Also, output containers (i.e. mariner2/container ) can be signed. This feature would give dalec spec writers control over certain aspects of signing. A rough abstraction would be:

• Provide the authority that will sign
• Provide the means by which signing will occur (cosign)
• Provide what will be signed
  • individual artifacts? binaries, tarballs, etc
  • sources and/or source tarballs produced for rpm builds?
  • output containers?

[cpuguy83]

This was completed in #214