Add support for signing images with cosign

[axtloss]

View Vanilla-OS/ABRoot#261 for more details

[xynydev]

This is something that needs to be done in CI / GitHub Actions after the container image is built from the Containerfile built by Vib.

[kbdharun]

This is something that needs to be done in CI / GitHub Actions after the container image is built from the Containerfile built by Vib.

Yep, the sigstore's cosign action should be run only after pushing the image to the respective registry.

[axtloss]

That doesn't mean it's something vib can't handle.

[axtloss]

With #76 being merged this only requires a finalize plugin