feat: initial implementation of gateway-cert-operator

Kubernetes operator that automates HTTPS listener configuration on
Gateway API Gateway resources whenever a cert-manager Certificate is
created or updated.

Core behaviour:
- Watches cert-manager Certificate resources for the annotation
  gateway-cert-operator.io/gateway-name to identify the target Gateway
- Builds HTTPS listeners (prefixed "auto-") from each Certificate's
  DNS SANs and merges them into the target Gateway's listener list
- Preserves any manually-managed listeners; removes stale auto-listeners
  when Certificates are deleted or their annotations are removed
- Supports optional annotations to override the target namespace and
  listener port (default 443)

Components:
- main.go                            – manager setup, scheme registration,
                                       health/readiness probes
- internal/controller/               – Certificate reconciler with field
                                       indexing and dual-watch pattern
- internal/gateway/patch.go          – listener construction, merge, and
                                       equality helpers
- deploy/manifests.yaml              – Namespace, RBAC, and Deployment
- docs/README.md                     – usage guide and architecture notes
- Dockerfile                         – distroless multi-stage build

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go.mod

@@ -0,0 +1,69 @@
+module github.com/example/gateway-cert-operator
+
+go 1.25.0
+
+require (
+	github.com/cert-manager/cert-manager v1.20.0
+	go.uber.org/zap v1.27.1
+	k8s.io/apimachinery v0.35.2
+	k8s.io/client-go v0.35.2
+	sigs.k8s.io/controller-runtime v0.23.3
+	sigs.k8s.io/gateway-api v1.5.1
+)
+
+require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
+	github.com/emicklei/go-restful/v3 v3.13.0 // indirect
+	github.com/evanphx/json-patch/v5 v5.9.11 // indirect
+	github.com/fsnotify/fsnotify v1.9.0 // indirect
+	github.com/fxamacker/cbor/v2 v2.9.0 // indirect
+	github.com/go-logr/logr v1.4.3 // indirect
+	github.com/go-logr/zapr v1.3.0 // indirect
+	github.com/go-openapi/jsonpointer v0.22.4 // indirect
+	github.com/go-openapi/jsonreference v0.21.4 // indirect
+	github.com/go-openapi/swag v0.23.1 // indirect
+	github.com/go-openapi/swag/jsonname v0.25.4 // indirect
+	github.com/google/btree v1.1.3 // indirect
+	github.com/google/gnostic-models v0.7.1 // indirect
+	github.com/google/go-cmp v0.7.0 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/josharian/intern v1.0.0 // indirect
+	github.com/json-iterator/go v1.1.12 // indirect
+	github.com/mailru/easyjson v0.9.1 // indirect
+	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
+	github.com/modern-go/reflect2 v1.0.3-0.20250322232337-35a7c28c31ee // indirect
+	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
+	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
+	github.com/prometheus/client_golang v1.23.2 // indirect
+	github.com/prometheus/client_model v0.6.2 // indirect
+	github.com/prometheus/common v0.66.1 // indirect
+	github.com/prometheus/procfs v0.17.0 // indirect
+	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/x448/float16 v0.8.4 // indirect
+	go.uber.org/multierr v1.11.0 // indirect
+	go.yaml.in/yaml/v2 v2.4.3 // indirect
+	go.yaml.in/yaml/v3 v3.0.4 // indirect
+	golang.org/x/net v0.51.0 // indirect
+	golang.org/x/oauth2 v0.35.0 // indirect
+	golang.org/x/sync v0.19.0 // indirect
+	golang.org/x/sys v0.41.0 // indirect
+	golang.org/x/term v0.40.0 // indirect
+	golang.org/x/text v0.34.0 // indirect
+	golang.org/x/time v0.14.0 // indirect
+	gomodules.xyz/jsonpatch/v2 v2.5.0 // indirect
+	google.golang.org/protobuf v1.36.11 // indirect
+	gopkg.in/evanphx/json-patch.v4 v4.13.0 // indirect
+	gopkg.in/inf.v0 v0.9.1 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+	k8s.io/api v0.35.2 // indirect
+	k8s.io/apiextensions-apiserver v0.35.2 // indirect
+	k8s.io/klog/v2 v2.140.0 // indirect
+	k8s.io/kube-openapi v0.0.0-20260127142750-a19766b6e2d4 // indirect
+	k8s.io/utils v0.0.0-20260210185600-b8788abfbbc2 // indirect
+	sigs.k8s.io/json v0.0.0-20250730193827-2d320260d730 // indirect
+	sigs.k8s.io/randfill v1.0.0 // indirect
+	sigs.k8s.io/structured-merge-diff/v6 v6.3.2 // indirect
+	sigs.k8s.io/yaml v1.6.0 // indirect
+)