The autoinstall schema for "apt" now supports the "preferences"
configuration. Each preference element should contain the properties
described below, that 1:1 map with the keywords from apt_preferences(5):
* package <-> Package:
* pin <-> Pin:
* pin-priority <-> Pin-Priority:
These preferences are forwarded to curtin through
subiquity-curtin-apt.conf. Support for these rules must be added to
curtin as well.
Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>