command to list available local images

It would be nice if there was a command to list the local images `multipass images` instead of having to run `multipass info ` and copy the hash.

Adding that those images get purged after N days, the selected hash may not be available anyways and it'll start the image download process.

Start by tracing the existing `multipass info ` command and the local-image lookup path. Define the `multipass images` entry point so it lists available local images and makes clear when an image has been purged or must be downloaded; verify the command through the existing CLI test path if one covers `multipass info`.

Written by the indexing model from the issue text.