feat: Add raw diff API endpoint

Gary Holtzrequested to merge
gmh-add-raw-diff-api-endpoint into main

Adding the new raw diff API endpoint - /projects/:id/merge_requests/:merge_request_iid/raw_diffs

Introduced in Add `raw_diff` option to list merge request dif... (gitlab-org/gitlab!178813 - merged)

Edited by Gary Holtz

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