Add package to manipulate gemnasium-db repo

What does this MR do?

Add a package to manipulate a git clone of gemnasium-db:

• add Advisory, used to decode YAML advisories
• add Repo, used to list YAML advisories

This MR does NOT cover:

• synchronizing local gemnasium-db directory with remote gemnasium-db repo
• adding gemnasium-db to the Docker image
• evaluating affected ranges

What are the relevant issue numbers?

gitlab-org/gitlab#14630 (closed)