Source downloader for simple streams

It would be nice to be able to do something like

```yaml
source:
  downloader: simplestreams-http
  url: https://images.linuxcontainers.org
  product: ubuntu:noble:amd64:cloud
```

to build on top of an existing image similarly to how Docker `FROM` works. Using `rootfs-http` theoretically works as it is, but a separate downloader would be cleaner.