Add central workflow which requires CORE4 labels on PRs prior to merging. (#3272)

Author: Reettaphant

.github/workflows/check-labels.yaml

@@ -0,0 +1,9 @@
+name: CORE4 label enforcement
+permissions:
+  contents: read
+on:
+  pull_request:
+    types: [opened, labeled, unlabeled, synchronize, reopened, edited]
+jobs:
+  require-label:
+    uses: guardian/.github/.github/workflows/require-label.yaml@4cb5024736632ffcc564b7f4b772c38b8e5ce739 #  v2.0.0