feat: cosign multi arch

Presentation

Closes #142 (closed)

• fix(cosign): publish all multi-arch attachments
• fix(cosign): sign all images in multi-arch
• feat(cosign): split sign and attest opts

Checklist

• Documented:
  • README.md documents the new feature
  • kicker.json describes the new feature
• Tested & examplified:
  • multi-arch: https://gitlab.com/to-be-contained/images/-/pipelines/2009739916
  • single-arch: https://gitlab.com/to-be-contained/images/-/pipelines/2009746068