feat: Multi-arch (amd64 + arm64) container images for machine-controller and OSM

[mfahlandt]

Summary

The machine-controller and operating-system-manager currently only ship amd64 container images. This blocks full ARM64 support in KubeOne (and KKP) across all providers.

Current State

• machine-controller image: amd64 only
• operating-system-manager image: amd64 only
• KubeOne arm64 binary exists but can't fully provision ARM64 clusters without these images

Proposal

Build and publish multi-arch (amd64 + arm64) images for both:

• machine-controller
• operating-system-manager

GitHub Actions supports multi-arch builds natively via docker buildx — this should be achievable without additional infrastructure.

Relates to

• ARM support for Hetzner doesn't work with machine-controller kubeone#2922 — ARM support blocked by missing MC arm64 image

Desired Outcome

• CI pipeline produces multi-arch manifests (amd64 + arm64) for MC and OSM
• Existing amd64 users unaffected (same image tags, manifest list)
• ARM64 clusters can provision worker nodes via MC/OSM

/kind feature