Add a workflow to check required labels

To facilitate generating release note and backporting, all PRs should be
labelled properly with the following up actions. However, PRs are merged
without any label in some cases. The workflow checks there is at least
one action label is added before a PR can be merged to remind that. If
there is no required action, "action/none" should be labelled
explicitly.

Signed-off-by: Quan Tian <qtian@vmware.com>

.github/workflows/label.yml

@@ -0,0 +1,14 @@
+name: Pull Request Labels
+on:
+  pull_request:
+    types: [opened, labeled, unlabeled, synchronize]
+jobs:
+  label:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: mheap/github-action-required-labels@v4
+        with:
+          mode: minimum
+          count: 1
+          labels: "action/backport, action/release-note, action/none"
+          add_comment: true