This second milestone introduces artifact handling on the debusine server. Users should be able to create artifacts and upload files into them. Then use those artifacts as input for work requests that they submit.

Workers should be able to retrieve those user-supplied artifacts and also create new artifacts as result of the tasks that they are executing.

By default all those artifacts and files are private but they can be retrieved through extra-permissions that are granted for the benefit of the work request. The created artifacts and files should be owned by the user who submitted the work request.