[Feature] Add support for conffiles

[hector-cao]

Configuration files of a package are special and so need special treatment at package installation and upgrades.

They are part of the package control data and managed by the Debian package management tool:

https://man7.org/linux/man-pages/man5/deb-conffiles.5.html

debcraft need to allow developers to specify a list of configuration files for each binary package and
list them in the conffiles that is part of the package control metadata.

[cmatsuoka]

Partial supported added in #156, more PRs to follow.