This change adds support for configuration of Apt package manager in
/etc/apt/apt.conf.d/. This allows adding arbitrary global configuration
options for Apt. Options can be added in different files, allowing for
different filename-based priorities.

CI tests and documentation are provided.

Story: 2009655
Task: 43987

Change-Id: I9d7d18851359e97cd01b4c2287bf79110796b25a