ImageVolume with an image digest

<!-- This section is incredibly important for producing high-quality, user-focused documentation such as release notes or a development roadmap. It should be possible to collect this information before implementation begins, in order to avoid requiring implementors to split their attention between writing release notes and implementing the feature itself. KEP editors and SIG Docs should help to ensure that the tone and content of the Summary section is useful for a wide audience. A good summary is probably at least a paragraph in length. Both in this section and below, follow the guidelines of the [documentation style guide]. In particular, wrap lines to a reasonable length, to make it easier for reviewers to cite specific portions, and to minimize diff churn on updates. [documentation style guide]: https://github.com/kubernetes/community/blob/master/contributors/guide/style-guide.md --> The ImageVolume feature, implemented in KEP-4639, introduces a new way to define volumes for pods using a container image as the source. The image can be specified by its name and tag, or by its digest, alongside a pull policy, just like a container image is being specified in a pod spec. However, a difference between ImageVolume and a "standard" container image is that the image digest for container images is advertised in the pod's status as part of the containerStatuses field. As opposed to that, the digest of the image used for an ImageVolume is not advertised in the pod's status. This KEP aims to close this gap, by adding a new field to the pod's status that will contain the digest of the image.