ci: Add scheduled job to cleanup resources

This is the first part of adding a job to clean up potentially dangling
Azure resources. This will be based on Jeremi's tool from
https://github.com/jepio/kata-azure-automation.

At first, we'll only clean up AKS clusters, as this is what has been
causing us problems lately, but this could very well be extended to
cleaning up entire resource groups, which is why I left the different
names pretty generic (i.e. "resources" instead of "clusters").

Signed-off-by: Aurélien Bombo <abombo@microsoft.com>

.github/workflows/cleanup-resources.yaml

@@ -0,0 +1,13 @@
+name: Cleanup dangling Azure resources
+on:
+  schedule:
+    - cron: "0 */6 * * *"
+  workflow_dispatch:
+
+jobs:
+  cleanup-resources:
+    runs-on: ubuntu-latest
+    secrets: inherit
+    steps:
+      - name: Cleanup resources
+        run: python3 tests/cleanup_resources.py