[Feature]: Allow custom ServiceAccount for DCGM Exporter DaemonSet

Problem

GPU Operator hardcodes the DCGM Exporter ServiceAccount to nvidia-dcgm-exporter (assets + DaemonSet). DCGMExporterSpec has no serviceAccount / serviceAccountName field, and TransformDCGMExporter never overrides it. Helm/ClusterPolicy cannot change it; patches are reverted on reconcile.

Request

Add ClusterPolicy/Helm support to configure the DCGM Exporter SA, with default behavior unchanged:

dcgmExporter:
  serviceAccount:
    create: false                 # true = operator creates SA (current behavior)
    name: my-metrics-identity     # default: nvidia-dcgm-exporter

• Default unset → keep creating/using nvidia-dcgm-exporter (backward compatible).
• create: false → use an existing SA in the operator namespace; bind Role/ClusterRoleBindings to that SA.
• Document that SAs are namespaced (cross-namespace SA is out of scope).

Why

Enterprise platforms bind identity/PKI/IRSA/Workload Identity to a specific SA name. Without this, users must run a standalone exporter, vendor-patch the operator, or use fragile admission mutators.

Pros

• Keep using the operator-managed DCGM Exporter (no parallel DaemonSet).
• Fit org identity/IAM/PKI SA naming requirements.
• Align with existing per-component knobs (dcgmExporter.annotations, env, serviceMonitor).
• Backward compatible; reduces forks/mutators/reconcile fights.

References

• https://github.com/NVIDIA/gpu-operator/blob/v26.3.3/assets/state-dcgm-exporter/0100_service_account.yaml
• https://github.com/NVIDIA/gpu-operator/blob/v26.3.3/assets/state-dcgm-exporter/0800_daemonset.yaml

Start with DCGMExporterSpec and TransformDCGMExporter, then inspect assets/state-dcgm-exporter/0100_service_account.yaml and 0800_daemonset.yaml. Trace the ClusterPolicy and Helm configuration path, including the Role and ClusterRoleBinding resources. Done means custom names and create behavior work in the operator namespace, while an unset configuration preserves the existing ServiceAccount behavior and the namespaced limitation is documented.