Extend internal API to download report-type artifacts (e.g. junit.xml, codeclimate.json, etc)

Problem to Solve

Currently, users can download archive-type artifacts through GitLab API.

In terms of Extend artifacts definition to provide reports: context , we should extend API to let users to download report-type artifacts, too.

This item is needed to unblock the security team as per https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7417#note_104271725.

This also unblocks https://gitlab.com/gitlab-org/gitlab-ee/issues/6061

Proposal

Extend https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/job_artifacts.rb