annotation support

[eugene-eeo]

same use case as #1090, pushing to ghcr registry. this PR adds ability to set annotations for both single/multi-arch builds, so they are nicely labelled on ghcr.io:

ko build --tags=v0.0.1 \
    --sbom=none --platform linux/amd64,linux/arm64 \
    --image-annotation org.opencontainers.image.source='https://github.com/foo/bar' \
    --image-annotation org.opencontainers.image.description='something about the package' \
    --push=true --bare \
    .

references:

• https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#labelling-container-images
• https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#adding-a-description-to-multi-arch-images

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[Qup42]

It would be really great to have this feature. I would love to build properly annotated multi-arch images with a downstream tool (goreleaser).