Add Release Links to the API

It would be nice, if the API included [release links](https://docs.gitlab.com/ee/api/releases/links.html).

If they already exist, my bad.

  - python-gitlab version: 2.6.0
  - API version you are using (v3/v4): v4
  - Gitlab server version (or gitlab.com): internal (gitlab CE 13.7.1)
