Implement update-collection-with-artifacts event reaction

On top of #337 (closed) and !712 (merged), we need to implement the update-collection-with-artifacts action, specified in https://freexian-team.pages.debian.net/debusine/reference/devel-blueprints/workflows.html#update-collection-with-artifacts.

Note that we might want to adjust the way collections are specified there based on the discussion in #338 (closed), but it's fine to go with what the current specification says for now. An early use case will likely be adding output artifacts to the workflow's internal_collection.