Add Git push option to merge when pipeline succeeds

What does this MR do?

Support setting a merge request to merge when the pipeline succeeds with git push options.

This is a small iteration that builds on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26752.

To create a new merge request and set it to merge when the pipeline succeeds:

 git push -u origin -o merge_request.create \
  -o merge_request.merge_when_pipeline_succeeds

To update an existing merge request to set it to merge when the pipeline succeeds:

git push -u origin -o merge_request.merge_when_pipeline_succeeds

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/53198

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #53198 (closed)