Add custom ServingRuntime overrides: image, protocolVersions, imagePullSecrets

[tartkri-bot]

Problem

The predictor topology supports runtimeVersion and runtime (by name), but does not allow:

1. Custom runtime image: Override the image URI of the selected ServingRuntime per Instance.
2. Custom protocol versions: Specify protocolVersions (e.g., only v2 for gRPC, only v1 for REST).
3. Image pull secrets: imagePullSecrets for pulling from private registries.
4. Resource limits on init containers: The storage initializer init container runs with default resource requests; users may want to tune it or the model server's resources independently.

This is especially relevant for teams who:

• Maintain a patched/custom build of Triton / vLLM / sklearn.
• Use private registries (e.g., Harbor, ECR with pull-through cache).
• Need specific protocol versions for their client SDKs.

Suggested approach

Add to ModelServerCustomSpec:

• runtimeImage — string, overrides the runtime image for this instance.
• protocolVersions — []string (e.g., ["v1"], ["v1", "v2"]).
• imagePullSecrets — []corev1.LocalObjectReference.
• storageInitializerResources — corev1.ResourceRequirements (optional, to tune the init container).

For LLM topology, add equivalent fields or document how to achieve this via the config escape hatch.

Acceptance criteria

• runtimeImage overrides the KServe ServingRuntime image for the predictor.
• protocolVersions restricts the serving protocol.
• imagePullSecrets is propagated to the InferenceService spec.
• storageInitializerResources is propagated when disableStorageInitializer is false.
• Chainsaw tests cover custom image override and imagePullSecrets.