Use the default username as provided by `cloud-init`

To keep as close as possible with cloud deployments, we were asked to use the default username when issuing `multipass shell`, as provided by `cloud-init`.

It depends on the image being launched ([cloud init's template](https://github.com/cloud-init/cloud-init/blob/master/config/cloud.cfg.tmpl)), so we might need to have our own "service" user and use that to find out what user we should shell into.

Start with the `multipass shell` entry point and the cloud-init config template linked in the issue to understand how the image's default username is selected. Determine how Multipass can identify that username, including whether a service user is needed. Done means shell opens as the cloud-init-provided default user across supported images.

Written by the indexing model from the issue text.