[action] Add needs triage labeler for pull requests

.github/needs-triage-labeler.yml

+needs triage:
+- "**"

.github/workflows/labeler.yml

+name: labeler
+on:
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  label:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/labeler@v2
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"
+        configuration-path: .github/needs-triage-labeler.yml