API for get and add MR to merge train is missing

[Toni Neubert]

Description of the problem, including code/CLI snippet

The API for both "new" merge-train features is missing:

• Get the status of a merge request on a merge train
• Add a merge request to a merge train

Could you please add them?

Specifications

• python-gitlab version: 4.4.0
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): >= 15.11