feat(rbac): grant inference-operator access to InferenceGatewayConfig

[XuanCS]

What

Grant the inference-operator ClusterRole access to InferenceGatewayConfig:

• inferencegatewayconfigs: full CRUD (get;list;watch;create;update;patch;delete) — the operator upserts a scheduler entry on the shared gateway CR when a model opts in.
• inferencegatewayconfigs/status: get only — the operator mirrors gateway state onto the model's status; the SageMaker Inference Gateway controller owns status writes.

No /finalizers verb: the operator never finalizes the IGC (the gateway owns its lifecycle).

Why

Follow-up to #461, which added the spec/status.inferenceGateway opt-in fields to the JumpStartModel / InferenceEndpointConfig CRDs and bumped the operator to v3.6, but did not grant RBAC for the InferenceGatewayConfig resource.

When a model sets spec.inferenceGateway.enabled: true, the operator's first Get/Create/Update on the cluster InferenceGatewayConfig is rejected with forbidden without these verbs, so operator↔gateway wiring never completes. The operator interacts with the IGC via unstructured (GVK + field paths), so no cross-repo Go/type dependency is introduced — RBAC is the only thing needed at the Kubernetes API layer.

Scope

RBAC only; no chart/appVersion/image bump. The IGC CRD itself is installed by the sagemaker-inference-gateway chart.

Notes

Mirrors the internal operator ClusterRole (kubebuilder markers) that already carries these rules. Minimal-privilege split (status get only) is intentional.