Add support for triggering a project mirroring process

Signed-off-by: Dowideit, Sven (O&amp;A, St. Lucia) <Sven.Dowideit@csiro.au>

projects.go

@@ -1342,3 +1342,27 @@ func (s *ProjectsService) ChangeAllowedApprovers(pid interface{}, opt *ChangeAll
 
 	return pa, resp, err
 }
+
+// StartPullMirroringProject Start the pull mirroring process for a Project.
+//
+// GitLab API docs:
+// https://docs.gitlab.com/ee/api/projects.html#start-the-pull-mirroring-process-for-a-project-starter
+func (s *ProjectsService) StartMirroringProject(pid interface{}, options ...OptionFunc) (*Response, error) {
+	project, err := parseID(pid)
+	if err != nil {
+		return nil, err
+	}
+	u := fmt.Sprintf("projects/%s/mirror/pull", pathEscape(project))
+
+	req, err := s.client.NewRequest("POST", u, nil, options)
+	if err != nil {
+		return nil, err
+	}
+
+	resp, err := s.client.Do(req, nil)
+	if err != nil {
+		return resp, err
+	}
+
+	return resp, err
+}