feat(gateway-api): expose dsh-gateway externally via Traefik Gateway API (#12)

Add Gateway and HTTPRoute manifests to route external traffic to the
dsh-gateway service (dsh-demo:8080).

- Gateway dsh-gateway: HTTPS listener with TLS termination against a
  dsh-demo-tls cert secret.
- HTTPRoute dsh-gateway: hostname -> dsh-gateway:8080.
- Example TLS secret manifest + README with apply/verify/rollback.

Co-authored-by: SREDevOpsAgents <agents@sredevops.org>

gateway-api/dsh-demo/gateway.yaml

@@ -0,0 +1,20 @@
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  name: dsh-gateway
+  namespace: dsh-demo
+  labels:
+    app.kubernetes.io/part-of: dsh-demo
+spec:
+  gatewayClassName: traefik
+  listeners:
+    - name: https
+      port: 8443
+      protocol: HTTPS
+      tls:
+        mode: Terminate
+        certificateRefs:
+          - name: dsh-demo-tls
+      allowedRoutes:
+        namespaces:
+          from: Same