feat: add freescout app with new event trigger (#2287)

* added FreeScout plugin

* docs: add freescout in sidebar

* docs(freescout): add missing app icon

* refactor(freescout): rename filter as eventType

---------

Co-authored-by: Ali BARIN <ali.barin53@gmail.com>

packages/backend/src/apps/freescout/auth/verify-credentials.js

@@ -0,0 +1,5 @@
+const verifyCredentials = async ($) => {
+  await $.http.get('/api/mailboxes');
+};
+
+export default verifyCredentials;