[Feature] Add image cache management commands and speed up image downloads with torrent client

I just had an idea as I was experimenting with multipass yesterday. It would be pretty cool if you had the choice to use torrents to download operating system images, as this is typically much faster than a direct HTTP or FTP download.

In addition, it would be cool to have image cache management commands to download, list, purge, and search for remote images.

Here's how I would envision exposing the torrent option:

# Download an image, without creating a virtual machine, using torrent client
multipass download ubuntu-lts --torrent

# Passes through --torrent parameter to the download command above
multipass launch --torrent 

Thoughts?