Work with APT developers so that a Release file can contain an EOL date

Quite a few LTS users would have liked a warning displayed by APT a few months prior to the LTS EOL. To achieve this, it should be possible to define an EOL date in a Release file and have APT display a warning message linking to an URL that is also provided in the Release file so that people can have more details about the EOL and possible alternatives for after the EOL.