Managing the "known advisories" in code is a fairly slow means to make
changes, and

Instead, we can migrate the known advisories to a -contrib repo for more
community-provided contributions, without waiting for a full release
cycle to fetch the new updates.

To manage this, we can add the `contrib` subcommand, which right now
just allows fetching the latest version of the repository, which we'll
use a GitLab Go client for, as it simplifies the work we want to do, and
allows for us to stream directly to a file.

We then arbitrarily execute the SQL that is available in file(s) in the
`contrib/advisories` directory, allowing for longer-term usage.

Closes #140.