[enhancement]: Add support for exherbo distro

# Enhancement

[Exherbo](https://www.exherbolinux.org/) was initially a fork from Gentoo, nowadays it's very different. It is currently used in cloud infrastructure, and it would be a great feature to have it as part of the cloud init project.

In short, `exherbo` uses `paludis` as a package manager, the command line for it is `cave`. Public repositories are available on the public [gitlab](https://gitlab.exherbo.org/exherbo/arbor). It's not that different from other distros, but it would require to have some implementation of `paludis`.