Use a containerd plugin for side-loading images

[neoaggelos]

Summary

Add a containerd plugin that sideloads images when containerd starts (and, optionally, at a refresh interval)

The plugin can be configured by a section like this in the config.toml file of containerd (or in a separate file):

[plugins."io.containerd.service.v1.sideload-images"]
  disabled = true 
  sources = ["path1", "path2"]
  namespace = "k8s.io"

The default behaviour is to attempt to sideload any tar files found in /var/snap/k8s/common/images when containerd starts.

Changes

• Add patch for containerd plugin. This plugin is based on https://github.com/canonical/microk8s/blob/7a788d6d308672b0bde609e777727ded4255c134/build-scripts/components/containerd/patches/default/0001-microk8s-sideload-images-plugin.patch
• Also regctl under tools, such that we can easily run commands like regctl image export $image > image.tar

[bschimke95]

LGTM