Add shadow reviewer when PR's review is requested (#2505)

ready_for_review triggers the GitHub action only when a PR is moved from draft PR to "Ready to Review" stage.
So, shadow-reviewers don't get added when a PR is directly created without going through
the draft stage. Use review_requested to add shadow reviewer when a review is requested.

.github/workflows/shadow.yml

@@ -3,7 +3,7 @@ name: "Shadow reviews"
 on:
   pull_request:
     types: 
-    - ready_for_review
+    - review_requested
     branches:
       - master