github: Create size label for every PR

Examples: size/XS, size/S, size/M, size/L, ...

Inspired by kubernetes/kubernetes.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

.github/workflows/size-label.yml

@@ -0,0 +1,25 @@
+name: Add PRs size label
+
+on: pull_request_target
+
+jobs:
+  size-label:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+      - name: size-label
+        uses: "pascalgn/size-label-action@v0.4.2"
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+        with:
+          sizes: >
+            {
+              "0": "XS",
+              "20": "S",
+              "50": "M",
+              "200": "L",
+              "800": "XL",
+              "2000": "XXL"
+            }