refactor: Use beforeRequest hook with slack app

packages/backend/src/apps/slack/index.ts

@@ -1,4 +1,7 @@
-export default {
+import defineApp from '../../helpers/define-app';
+import addAuthHeader from './common/add-auth-header';
+
+export default defineApp({
   name: 'Slack',
   key: 'slack',
   iconUrl: '{BASE_URL}/apps/slack/assets/favicon.svg',
@@ -6,4 +9,6 @@ export default {
   supportsConnections: true,
   baseUrl: 'https://slack.com',
   apiBaseUrl: 'https://slack.com/api',
-};
+  primaryColor: '4a154b',
+  beforeRequest: [addAuthHeader],
+});