ci: add workflow to label PR size

Lately the volume of PRs has increased and it is difficult to keep up.
Also a few PRs have come through with huge changes to DEVICES.md due to
bad conflict resolution, which is a red flag.

Labelling the PRs based on size lets the maintainers prioritise the
easy reviews instead of getting stuck on difficult to review changes
and also automatically flags those mistaken huge changes to let the
author hopefully deal with it before the maintainers have to review it.

.github/workflows/label-pr-size.yml

@@ -0,0 +1,15 @@
+name: Label PRs based on size
+
+on: [pull_request]
+
+jobs:
+  add_pr_size_label:
+    runs-on: ubuntu-latest
+    name: Check PR size
+
+    steps:
+      - name: Label and comment PR
+        uses: boschresearch/pr-size-labeler@v5
+        # Uncomment these lines if you want to use your own custom configuration
+        # with:
+        #   bucketConfigFile: "<PATH_TO_YOUR_BUCKET_CONFIG>"