code: public List<GitlabBuildVariable> getBuildVariables(Integer projectId)
              throws IOException {
          String tailUrl = GitlabProject.URL + "/" + projectId + GitlabBuildVariable.URL;
          GitlabBuildVariable[] variables = retrieve().to(tailUrl, GitlabBuildVariable[].class);
          return Arrays.asList(variables);
      }
docs: Gets build variables associated with a project.
  
  @param projectId The ID of the project.
  @return A non-null list of variables.
  @throws IOException on gitlab api call error