Extending artifacts API to allow upload

Description

We have good API coverage for artifacts (https://docs.gitlab.com/ee/api/jobs.html#get-job-artifacts) but we don't have a public API for uploading and storing properly.

Proposal

Add API support for artifact upload. It should also support it when the setup includes direct upload to S3 by the runner (https://gitlab.com/gitlab-org/gitlab-ee/issues/2348).