Add Support for apt_auth.conf for PRODUCT_EXTRA_SOURCES

[CanNuhlar]

Currently isar expects an anonymous apt repository to be able to fetch custom debian package feed.

A mechanism that can use https://manpages.debian.org/testing/apt/apt_auth.conf.5.en.html is missing.

[amikan]

Hello @CanNuhlar,
On which side do you need to have repo authentication: on the target itself or during the build?
If first, you can just create a package with some /etc/apt/auth.conf or /etc/apt/auth.conf.d/*.conf to be installed on the target.
If second, you can put credentials in your sources.list like deb http://<user>:<password>@your.server/customdistro etc or with DISTRO_APT_PREMIRRORS hack described in https://groups.google.com/g/isar-users/c/XdTwvJHV07I/m/xwviSvQLAgAJ

P.S. what is PRODUCT_EXTRA_SOURCES?