Description of the problem, including code/CLI snippet

Add support for downloading a single artifact based on branch name

https://docs.gitlab.com/ce/api/jobs.html#download-a-single-artifact-file-from-specific-tag-or-branch

Specifications

• python-gitlab version: 1.9.0
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): 12.0.0