Support sha references to scylla images

[mflendrich]

Issue originally authored by tnozicka as #821

What should the feature do:
Currently scylla images can be specified only with a tag that has to be a semver. This dosn't work for named references like latest, rolling update references like 4.4 (sort of) and for sha references. As images can be overwritten sha reference is the only reliable way to always reference the same image.

As we need to know the information in the operator we can't just download and run the image to find out but when buliding the image we could set an image label that the operator would retrieve. The label is stored in the image manifest that can be downloaded separately from a container registry at low cost and the result can be cached.

Needs cooperation from scylla.

(Look at libs like github.com/containers/image/v5 )

[mflendrich]

Unclear if this requires any work. If it does, that work will be done as part of https://github.com/scylladb/scylla-operator-release/issues/377.