ci: assigner: Skip assignment for draft pull requests

This commit updates the pull request assigner workflow to skip the
reviewers, assignee and labels assignment for the pull requests that
are marked as draft.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/assigner.yml

@@ -2,6 +2,11 @@ name: Pull Request Assigner
 
 on:
   pull_request_target:
+    types:
+    - opened
+    - synchronize
+    - reopened
+    - ready_for_review
     branches:
     - main
     - v*-branch
@@ -9,6 +14,7 @@ on:
 jobs:
   assignment:
     name: Pull Request Assignment
+    if: github.event.pull_request.draft == false
     runs-on: ubuntu-20.04
 
     steps: