KEP-3562: [OptimizationJob] Implement Secure Auth between Controller and gRPC Suggestion Service

### What you would like to be added?

Context:
The OptimizationJob controller communicates with the Optuna suggestion container over gRPC within the cluster namespace ([KEP-3562](https://github.com/kubeflow/trainer/pull/3565)).

Problem:
Unencrypted, unauthenticated gRPC traffic inside the cluster opens potential for unauthorized trial manipulation or data interception in multi-tenant environments.

Future Goals:

- Implement mutual TLS (mTLS) or token-based authentication between the controller/workers and the gRPC suggestion service.

- Inject certificates dynamically using standard Kubernetes secret/cert-manager integrations

Relates to KEP-3562 Master Tracking [Issue](https://github.com/kubeflow/trainer/issues/3562).

### Why is this needed?

Securing gRPC communication prevents unauthorized network entities from tampering with trial parameters or objective metrics.

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