Automated Update_Fetch image digests from registry

[souleb]

This is a followup to #4.

#4 enables fetching images from a Flux distribution github repository. But it should instead fetch images metadata directly from the registry. We already know the name and tag for every controller for a given release. What can really change and is needed is the digest. For different setup an advertised digest in github may not be available yet in the provided registry. And really the registry should be the source of truth for when to trigger an update.

[stefanprodan]

Closing as this doesn't work for immutable registries like AWS Marketplace ECR and others.