Allow artifacts to be uploaded to object store
Description
The runner should be able to upload artifacts to the object store, in the same way it can upload caches to it.
I have a number of builds that produce large artifacts (C++ compiled programs and libraries) that the normal gitlab web server simply can't handle, currently I work around this by compressing everything and uploading that compressed tarball as the artifact. However it'd be easier if I could just upload everything to S3.