Add merge pipeline to the merge requests API

Problem to solve

Spotted in !22292 (merged) :

@nick.thomas started a discussion: (+3 comments)

Should we add this to the graphql api at the same time to avoid having it fall further behind?

Further details

The merge_pipeline is not in the API. We do give out the merge_commit_sha, so I think it would be useful to add this attribute too.

Proposal

We should add it to both graphql and rest APIs at the same time.

What does success look like, and how can we measure that?

Able to get the merge pipeline for a merge request from the API

Links / references