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

Workflow Refactoring:

• Refactor monolithic ci.yaml into reusable workflow modules (variables.yaml, golang-checks.yaml, config-checks.yaml, image-builds.yaml, e2e-tests.yaml, release.yaml)

Forward Compatibility Testing:

• Add forward-compatibility.yaml workflow (weekly + manual trigger)
• Create get-latest-images.sh to fetch latest commit-based images with retry/backoff
• Create generate-values-overrides.sh to produce Helm values override file
• Use artifact-based values override approach (per @cdesiniotis review)

E2E Test Enhancements:

• Extend e2e-tests.yaml with optional use_values_override input
• Create env-to-values.sh to convert env vars to Helm values file
• Update install-operator.sh with proper YAML merging via yq
• Handle vGPU options correctly with values file approach
• Pass VALUES_FILE to remote test instances (per @rajathagasthya review)

Notifications:

• Add Slack notifications for test failures (JSON payload, configurable mentions)
• Guard notification step against missing secrets

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

Review Feedback Addressed

• Use values override file instead of individual input vars (@cdesiniotis)
• Move operator_image/operator_version to variables workflow (@cdesiniotis)
• Remove unnecessary workflow inputs from forward-compatibility (@cdesiniotis)
• Remove year from copyright headers (@rajathagasthya)
• Pass VALUES_FILE to remote test instance (@rajathagasthya)
• Fix Slack JSON payload format
• Use yq for proper YAML merging
• Handle vGPU with values file approach
• Add retry with exponential backoff for image verification
• Remove push trigger from e2e-tests (secrets not inherited)
• Restore golangci-lint-action to v9
• Write SSH key before SCP step
• Restore Holodeck to v0.2.18
• Remove unused operator_version input from release workflow
• Guard Slack notification against missing secrets