Support for new merge train APIs

p-varma

Description of the problem, including code/CLI snippet

Gitlab now support API for merge train to add a MR to merge train, list the status of MR in the merge train etc. These feature are currently missing in the code and should be added.

Expected Behavior

Actual Behavior

Specifications

python-gitlab version:
API version you are using (v3/v4): v4
Gitlab server version (or gitlab.com): 15.10.0