Add method to publish build artifacts to GitLab

There's currently a lack of a method to push build artifacts from Jenkins to GitLab as resulting from the builds.
On Merge Requests this can be hacked around by providing a comment with the artifact links, but for regular Push and Tag builds that method won't work.

GitLab does seem to have [support for this](https://gitlab.com/help/api/ci/builds.md#upload-artifacts-to-build) through their API as well, though I don't know how hard it would be to do from the Jenkins side.