KEP-3562: [OptimizationJob] Refactor Katib gRPC protocol contracts for stateless suggestion services

### What you would like to be added?

Context:
Historically, Katib relied on a specific gRPC interface for suggestion containers tied to its DB-backed architecture.

Problem:
`OptimizationJob` in `kubeflow/trainer` relies on a lightweight, stateless gRPC model ([KEP-3562](https://github.com/kubeflow/trainer/pull/3565)). The old Katib gRPC definitions carry unnecessary legacy state assumptions.

Future Goals:

Refactor the gRPC proto definitions (`api/v1alpha1/`) to cleanly decouple suggestion generation from persistence.

Establish a clean, lightweight protocol contract for requesting hyperparameter combinations and reporting trial metrics.

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

### Why is this needed?

Refactoring the gRPC protocol simplifies communication, reduces payload overhead, and allows suggestion backends to operate in ephemeral container lifecycle models without DB overhead.

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