feat(deploy): add Kustomize deployment option

[mkoushni]

[Feature] Enable deployment using Kustomize

Adds a Kustomize deployment path for the Inference Payload Processor (IPP),
aligning with the llm-d community deployment standard and enabling IPP to be
consumed as a step in llm-d-benchmark
and GitOps workflows that prefer Kustomize over Helm.

The existing Helm chart (config/charts/payload-processor/) is untouched;
Kustomize is an additive alternative.

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Fixes #125

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Changes

New: config/kustomize/

config/kustomize/
├── README.md                              ← full usage guide
├── base/                                  ← provider-agnostic core resources
│   ├── kustomization.yaml
│   ├── deployment.yaml                    ← Deployment (1 replica, port 9004/9005)
│   ├── service.yaml                       ← ClusterIP Service (HTTP2, port 9004)
│   ├── serviceaccount.yaml
│   ├── rbac.yaml                          ← Role + RoleBinding (single-namespace)
│   └── configmap.yaml                     ← default PayloadProcessorConfig
└── overlays/
    ├── default/kustomization.yaml         ← no provider (base only)
    ├── istio/                             ← + EnvoyFilter + DestinationRule
    │   ├── kustomization.yaml
    │   ├── envoyfilter.yaml
    │   └── destinationrule.yaml
    └── gke/                               ← + GCPRoutingExtension + HealthCheckPolicy
        ├── kustomization.yaml
        ├── gcproutingextension.yaml
        └── healthcheckpolicy.yaml

Each overlay exposes a single namespace: field as the sole namespace control
point — the base layer intentionally omits metadata.namespace so that the
overlay drives it.

Modified: Makefile

New ##@ Deployment section:

Target	Description
make kustomize	Download kustomize binary to bin/ if absent
make kustomize-build [KUSTOMIZE_OVERLAY=default|istio|gke]	Render manifests to stdout
make kustomize-deploy [KUSTOMIZE_OVERLAY=default|istio|gke]	Apply to cluster
make kustomize-undeploy [KUSTOMIZE_OVERLAY=default|istio|gke]	Remove from cluster

Modified: README.md

Added a Deployment section with side-by-side Helm and Kustomize quick-start
commands.

Fixed: pre-existing goconst lint issues

Extracted repeated string literals into named constants to clear golangci-lint
(4 issues, pre-existing):

File	Constants added
pkg/config/loader/defaults.go	configAPIVersion, configKind
test/integration/util.go	headerContentLength, bodyFieldPrompt

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Usage

# No provider (Deployment + Service only)
kubectl kustomize config/kustomize/overlays/default | kubectl apply -f -

# Istio (adds EnvoyFilter + DestinationRule)
make kustomize-deploy KUSTOMIZE_OVERLAY=istio

# GKE (adds GCPRoutingExtension + HealthCheckPolicy)
make kustomize-deploy KUSTOMIZE_OVERLAY=gke

# Custom namespace
cd config/kustomize/overlays/default
kustomize edit set namespace my-namespace

See config/kustomize/README.md for customization
options (namespace, image tag, custom IPP config, multi-namespace RBAC).

---

Testing

Kustomize build validation

All three overlays render without errors and produce the expected resource kinds:

Overlay	Resources
default	ConfigMap, Deployment, Role, RoleBinding, Service, ServiceAccount
istio	+ DestinationRule, EnvoyFilter
gke	+ GCPRoutingExtension, HealthCheckPolicy

Validated with:

kubectl kustomize config/kustomize/overlays/default
kubectl kustomize config/kustomize/overlays/istio
kubectl kustomize config/kustomize/overlays/gke

Go test suite

All 24 test packages pass with race detection enabled (go test -race -count=1 ./...):

Package	Result	Coverage
pkg/common	✅ PASS	81.8%
pkg/common/envoy	✅ PASS	87.5%
pkg/common/envoy/test	✅ PASS	100.0%
pkg/common/error	✅ PASS	100.0%
pkg/common/observability/logging	✅ PASS	69.4%
pkg/config/loader	✅ PASS	50.0%
pkg/datastore/inmemory	✅ PASS	100.0%
pkg/framework/interface/datalayer	✅ PASS	97.4%
pkg/framework/interface/requesthandling	✅ PASS	52.4%
pkg/framework/plugins/datalayer/notificationsource	✅ PASS	45.2%
pkg/framework/plugins/datalayer/requestmetadata	✅ PASS	92.3%
pkg/framework/plugins/modelselector/picker/maxscore	✅ PASS	66.7%
pkg/framework/plugins/modelselector/picker/random	✅ PASS	88.9%
pkg/framework/plugins/modelselector/picker/weightedrandom	✅ PASS	66.7%
pkg/framework/plugins/modelselector/scorer/costaware	✅ PASS	95.8%
pkg/framework/plugins/requesthandling/basemodelextractor	✅ PASS	81.4%
pkg/framework/plugins/requesthandling/bodyfieldtoheader	✅ PASS	100.0%
pkg/framework/plugins/requesthandling/profilepicker/single	✅ PASS	100.0%
pkg/handlers	✅ PASS	74.4%
pkg/metrics	✅ PASS	50.0%
pkg/modelselector	✅ PASS	82.4%
pkg/server	✅ PASS	61.1%
test/integration	✅ PASS	97.1%

Total coverage: 51.5% (statements)

Linting

golangci-lint run  →  0 issues.
go vet ./...       →  clean

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Release note (write NONE if no user-facing change):

Add Kustomize deployment support with base manifests and overlays for default,
Istio, and GKE providers. New Makefile targets: kustomize-build, kustomize-deploy,
kustomize-undeploy (KUSTOMIZE_OVERLAY=default|istio|gke).