code: public void createSubExtensions(Project project) {
      tools =
          ((ExtensionAware) this).getExtensions().create(TOOLS_EXT, ToolsExtension.class, project);
      deploy =
          ((ExtensionAware) this).getExtensions().create(DEPLOY_EXT, DeployExtension.class, project);
      stage =
          ((ExtensionAware) this)
              .getExtensions()
              .create(STAGE_EXT, StageAppYamlExtension.class, project);
    }
docs: Create nested configuration blocks as Extensions.