Update `Repository::ProjectPush` to implement `fabricate_via_api!`

The fabricate_via_api! methods was implemented in Resource::MergeRequest in !14336 (merged), but it depends on Repository::ProjectPush, which does not implement fabricate_via_api!.

fabricate_via_api! should be implemented in Repository::ProjectPush so that Resource::MergeRequest.fabricate_via_api! doesn't use the UI.