[CI] Auto-label new PRs according to draft status (#6361)

* add auto-label github action

* Add missing 'remove-labels' line

substrate/.github/workflows/auto-label-prs.yml

+name: Label new PRs
+on:
+  pull_request:
+    types: [opened,ready_for_review]
+
+jobs:
+  label-new-prs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Label new drafts
+        uses: andymckay/labeler@master
+        if: github.event.pull_request.draft == true
+        with:
+          add-labels: 'A3-inprogress'
+      - name: Label new PRs
+        uses: andymckay/labeler@master
+        if: github.event.pull_request.draft == false
+        with:
+          add-labels: 'A0-pleasereview'
+          remove-labels: 'A3-inprogress'