Become an ArtifactHub verified publisher and official images

[lbroudoux]

Integrate with cosign for signing container images as it has been done here https://github.com/microcks/microcks-hub/issues/63 for the operator images.

[lbroudoux]

Now done! Check it out here: https://artifacthub.io/packages/container/microcks-cli-image/microcks-cli