[Core] Add RBGStrategy workload strategy following OEP-0006

[diw-zw]

What this PR does

• Introduce a new RoleBasedGroup deployment mode and the corresponding RBGStrategy workload strategy that packs all InferenceService components (engine, decoder, router) into a single RoleBasedGroup custom resource
• Add RoleConfig / RoleConfigExtractor abstraction so each component (Engine, Decoder, Router) can produce its resolved pod spec and metadata without side effects, reusable by All-in-One workload strategies
• Implement RBGReconciler for create/update of the RBG CR with HPA-safe replica preservation, per-role HPA targeting RoleBasedGroupScalingAdapter, and component status propagation from RBG role statuses

Why we need it

Refer to OEP: #505

Changes

New files:

• pkg/controller/v1beta1/inferenceservice/components/role_config.go — RoleConfig struct, RoleConfigExtractor and ComponentStatusUpdater interfaces
• pkg/controller/v1beta1/inferenceservice/reconcilers/rbg/rbg_reconciler.go — RBG CR reconciler (create/update with replica preservation and label merging)
• pkg/controller/v1beta1/inferenceservice/workload/rbg_strategy.go — Full strategy: role config extraction, RBAC, RBG reconciliation, HPA, and status propagation
• Unit tests for both the reconciler and strategy

Modified files:

• constants.go — Add RoleBasedGroup deployment mode and RoleBasedGroupKind
• decoder.go, engine.go, router.go — Implement ExtractRoleConfig() and UpdateStatus()
• controller.go — Register RBG strategy (conditional on CRD availability), watch RoleBasedGroup resources
• cleanup.go — Include RBG in orphan resource cleanup
• hpa_reconciler.go — Add NewHPAReconcilerWithScaleTarget for custom scale target references
• status_util.go — Export SetComponentCondition for strategy-level status propagation
• main.go — Register rbgv1alpha2 scheme
• RBAC role YAMLs — Grant permissions for workloads.x-k8s.io/rolebasedgroups
• go.mod / go.sum — Add sigs.k8s.io/rbgs/api dependency (via github.com/sgl-project/rbg-api v0.7.0-alpha.3)

How to test

Checklist

• Tests added/updated (if applicable)
• Docs updated (if applicable)
• make test passes locally