feat(kubernetes): add core group with PVC + LoadBalancer Service [FIX-331]

[liamcervante]

Summary

First PR of the FIX-331 stack (add persistent storage + load balancer resources to the Kubernetes plugin). This lands the tree representation in go-proto; the parser and pricing changes follow in infracost/parser and infracost/providers.

Adds a Kubernetes core/v1 API group alongside the existing apps and batch groups, holding the two cost-relevant non-workload kinds:

• PersistentVolumeClaim — storage class + requested bytes (both price drivers, readable straight from the manifest) + annotations for cloud detection. The sizing lives in a shared StorageRequest type.
• StatefulSet.VolumeClaimTemplates — []core.StorageRequest, since a StatefulSet provisions one volume per template per replica (cost = Σ templates × replicas). Kept on the StatefulSet rather than as standalone PVCs so the replica multiplier isn't lost.
• Service — only type: LoadBalancer (the parser won't surface free ClusterIP/NodePort services); carries type, annotations (LB flavour + cloud signals) and ports.

Why a new group rather than extending workloads

PVCs and LoadBalancer Services aren't workloads (no containers/replicas), and — unlike per-workload compute — they map to discrete cloud resources (EBS/PD/Azure disk, ALB/NLB) provisioned outside the node pools. So pricing them is genuinely additive and doesn't double-count node-pool IaC.

Implementation notes

• Wiring rides entirely on the existing reflective walker (collectResources) and proto converter (convert.go) — no shared-code changes. New structs embedding resource.Resource are auto-discovered; the embedded StorageRequest is flattened the same way workload.Workload is.
• No hardcoded group/kind lists anywhere needed updating.

Testing

• Extended TestKubernetesRoundTrip to cover a standalone PVC (embedded-field flattening + tags + annotations), a LoadBalancer Service (type/annotations/ports), and StatefulSet volumeClaimTemplates.
• go build ./..., go vet, full go test ./..., and golangci-lint run all pass.

Technical docs

• No technical docs needed (internal tree type additions; no user-facing surface yet).