Built-in image copy mechanism (skopeo replacement)

`k8s-image-swapper` is currently using `skopeo`, a CLI, to copy images into the registry. This is being executed as an external command and can lead to several issues (OOM, security exposure via skopeo, ...).

`skopeo` is utilising `github.com/containers/image/v5/copy`, with some added mechanics like `retry`, which can be utilised directly in `k8s-image-swapper`.