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/httproute.yaml

@@ -0,0 +1,19 @@
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: dsh-gateway
+  namespace: dsh-demo
+  labels:
+    app.kubernetes.io/part-of: dsh-demo
+spec:
+  parentRefs:
+    - name: dsh-gateway
+      namespace: dsh-demo
+      sectionName: https
+  hostnames:
+    # TODO: replace with the actual public FQDN once provided.
+    - dsh-demo.example.com
+  rules:
+    - backendRefs:
+        - name: dsh-gateway
+          port: 8080