This change adds support for configuring Apt repositories on Ubuntu
hosts during host configuration.

Repositories are configured in a single file
(/etc/apt/sources.list.d/kayobe.sources), using the modern deb822
format [1]. This format is more flexible and readable than the original
single-line format, particularly if multiple options are used.

Using a single file allows us to more easily keep the set of
repositories in sync, since Ansible doesn't make it easy to clean things
up.

Support is added for marking repositories as signed by a particular GPG
key. This approach is now preferred over the deprecated [2] apt-key
tool, which resulted in a set of globally trusted keys.

It is also possible to disable the repositories in
/etc/apt/sources.list via apt_disable_sources_list. This allows for
replacing the standard repositories with a local mirror.

CI tests and documentation are provided.

[1] https://manpages.ubuntu.com/manpages/focal/en/man5/sources.list.5.html
[2] https://manpages.ubuntu.com/manpages/groovy/man8/apt-key.8.html

Story: 2009655
Task: 43818

Change-Id: I3f821937b0930a0ac9341178de7ae5123d82b957