Add weekly forward compatibility testing

[ArangoGutierrez]

Implement automated forward compatibility tests that validate GPU Operator against the latest published images from NVIDIA component repositories.

Changes:

• Add forward-compatibility.yaml workflow (weekly + manual trigger)
• Create get-latest-images.sh to fetch latest commit-based images
• Extend e2e-tests.yaml with optional component image inputs
• Update install-operator.sh to support device-plugin and mig-manager overrides
• Add Slack notifications for test failures
• Create variables.yaml as reusable workflow for shared CI variables

Components tested:

• ghcr.io/nvidia/container-toolkit
• ghcr.io/nvidia/k8s-device-plugin
• ghcr.io/nvidia/k8s-mig-manager

We could add other operands later, but I wanted to start with the core ones.

Schedule: Every Monday at 2 AM UTC