pkglists: update format to support URL and support

This adds additional fields with URL and boolean identifying supported lists.

Documentation and supported tags are just initial suggestion. The ideal would be to have documentation for all officially supported pkglists. I am also not exactly sure what we consider now as officially supported. But we can tweak that later.

Implements: https://gitlab.labs.nic.cz/turris/turris-os-packages/issues/378