Support Merge Train in Git Push Option

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Problem to solve

A few months back, we've introduced Git Push Options to trigger MWPS with git push e.g. git push -o merge_request.create -o merge_request.merge_when_pipeline_succeeds. This option currently doesn't support Merge Trains, but should be supported.

Similarly, we have #13993 (closed) as a missing path in API support for merge train.