Support managing login configurations in /etc/apt/auth.conf

Support managing login configurations in /etc/apt/auth.conf

APT 1.5 formally introduced support for specifying login configuration
settings (like username and password) for APT sources or proxies that
require authentication in the file `/etc/apt/auth.conf`. This file
follows a netrc-like format (like ftp or curl) and has restrictive
permissions. This is preferable to embedding login information directly
in sources.list entries (which are usually world-readable). See
https://manpages.debian.org/testing/apt/apt_auth.conf.5.en.html for
details.

This change adds a new, optional class parameter
`apt::auth_conf_entries` which expects an array of hashes (defined by a
new abstract data type `Apt::Auth_conf_entry`) that represent sets of
login configuration settings to record in `/etc/apt/auth.conf`. The
file's contents are rendered using a simple EPP template.

Contains updated spec tests and documentation.