Create a sample extra_apt_sources_list.yaml upon development setup

tools/init-dev.sh script can be updated so that when executed it also creates an extra_apt_sources_list.yaml in config directory with sample data, something as follows

bullseye-security:
  entry: deb-src http://security.debian.org/debian-security bullseye-security main
experimental:
  entry: deb http://deb.debian.org/debian experimental main
  allowed_users:
  - 122132
  - 4252