Setup assign-size-label action (#1762)

This replaces a bit of automation we were getting from the k8s-ci-robot

Author: alexeagle

.github/workflows/size-label.yml

@@ -0,0 +1,11 @@
+# See https://github.com/marketplace/actions/assign-size-label
+name: size-label
+on: pull_request
+jobs:
+  size-label:
+    runs-on: ubuntu-latest
+    steps:
+      - name: size-label
+        uses: "pascalgn/size-label-action@v0.4.2"
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"