Ensure image validation in imageService

Make sure image validation takes place right after pulling the image using imageService. 

• Have imageService.PullImage always return a digested reference to the result of the pull. Sascha’s PR has code for this.
• Add an imageService method to verify a signature on a (userSpecifiedImageName, StorageImageID). Exists in Sascha’s PR, actually that PR does rather more than necessary, especially the duplicate StorageImageID lookups must be dropped.
• Hook that new method into pullImageCandidate on paths where we have a local image with a StorageImageID. Obtain a Kubelet-usable “imageRef”.
• Make sure the current fallback way to obtain imageRef at the end of Server.pullImage is eliminated; imageRef should come from something that enforces signatures.

xref : https://issues.redhat.com/browse/RUN-1811?focusedId=23391305&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-23391305

This will help us validate an image right after pulling it for namespaced policies - https://redhat-internal.slack.com/archives/CK1AE4ZCK/p1712763142542329