ci: add GitHub Workflow to copy labels in Mergify created PRs

When Mergify creates a PR, the `ok-to-test` label needs to be added
before CI runs. Not all PRs need complete testing, and they may have
some `ci/skip/..` labels too. With this new GitHub Workflow, the labels
get copied from the original PR into the newly created PR.

See-also: Mergifyio/mergify#5088
Signed-off-by: Niels de Vos <ndevos@ibm.com>

.github/workflows/mergify-copy-labels.yaml

@@ -0,0 +1,16 @@
+---
+name: Mergify merge-queue labels copier
+# yamllint disable-line rule:truthy
+on:
+  pull_request_target:
+    types:
+      - opened
+
+jobs:
+  mergify-merge-queue-labels-copier:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Copying labels
+        uses: Mergifyio/gha-mergify-merge-queue-labels-copier
+#        with:
+#          labels: comma,separated,lists,of,labels,to,copy