Description of the problem, including code/CLI snippet

The Gitlab REST API offers a call to attempt to rebase the source_branch onto the target_branch. This hasn't been implemented in the Python Gitlab client library. I'd like to go ahead and submit an MR for this. However, I was wondering whether there's a reason this hasn't been implemented yet. If there are no objections from your side, I'd be fine creating an MR for this feature.

Specifications

• python-gitlab version: 1.8.0
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com):