Add ARM-backed AKS machine client

[bcho]

Summary

• Add an ARM-backed AKS Machine client and use it from start/daemon flows.
• Register the AKS Machine after local node startup while preserving E2E local-machine simulation behind the E2E build path.
• Remove cluster credential backfill and require kubelet cluster connection config to be provided.

Testing

• go test ./pkg/cmd/start ./pkg/aksmachine ./pkg/daemon ./pkg/config
• go test ./pkg/aksmachine ./pkg/cmd/daemon ./cmd/e2ehelper ./cmd/e2ehelper/localmachine
• go test -tags local_e2e ./pkg/aksmachine ./pkg/cmd/daemon ./cmd/e2ehelper ./cmd/e2ehelper/localmachine
• go build ./cmd/aks-flex-node
• go build -tags local_e2e ./cmd/aks-flex-node