Conformance: Add Port Support to Echo Server

[danehans]

Currently the echo server provides pod name, namespace, etc in the response headers. For use cases such as running multiple echo servers in a single pod, the echo server should optionally support echo'ing the HTTP server listening port number in the response headers. For example:

# Data Parallelism (DP) backend deployment: 3 pods, each listening on three ports to simulate vLLM ranks
apiVersion: apps/v1
kind: Deployment
metadata:
  name: dp-inference-model-server-deployment
  namespace: inference-conformance-app-backend
  labels:
    app: dp-inference-model-server
spec:
  replicas: 3
  selector:
    matchLabels:
      app: dp-inference-model-server
  template:
    metadata:
      labels:
        app: dp-inference-model-server
    spec:
      containers:
      - name: echoserver-3000
        image: gcr.io/k8s-staging-gateway-api/echo-basic:v20240412-v1.0.0-394-g40c666fd
        ports:
        - containerPort: 3000
        readinessProbe:
          httpGet:
            path: /
            port: 3000
          initialDelaySeconds: 3
          periodSeconds: 5
          failureThreshold: 2
        env:
        - name: HTTP_PORT # Default port for HTTP echo server
          value: "3000"
        - name: H2C_PORT # Default port for HTC echo server
          value: "3001"
        - name: POD_NAME
          valueFrom:
            fieldRef:
              fieldPath: metadata.name
        - name: NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace
        - name: POD_IP
          valueFrom:
            fieldRef:
              fieldPath: status.podIP
      - name: echoserver-3002
        image: gcr.io/k8s-staging-gateway-api/echo-basic:v20240412-v1.0.0-394-g40c666fd
        ports:
        - containerPort: 3002
        readinessProbe:
          httpGet:
            path: /
            port: 3002
          initialDelaySeconds: 3
          periodSeconds: 5
          failureThreshold: 2
        env:
        - name: HTTP_PORT
          value: "3002"
        - name: H2C_PORT
          value: "3003"
        - name: POD_NAME
          valueFrom:
            fieldRef:
              fieldPath: metadata.name
        - name: NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace
        - name: POD_IP
          valueFrom:
            fieldRef:
              fieldPath: status.podIP
      - name: echoserver-3004
        image: gcr.io/k8s-staging-gateway-api/echo-basic:v20240412-v1.0.0-394-g40c666fd
        ports:
        - containerPort: 3004
        readinessProbe:
          httpGet:
            path: /
            port: 3004
          initialDelaySeconds: 3
          periodSeconds: 5
          failureThreshold: 2
        env:
        - name: HTTP_PORT
          value: "3004"
        - name: H2C_PORT
          value: "3005"
        - name: POD_NAME
          valueFrom:
            fieldRef:
              fieldPath: metadata.name
        - name: NAMESPACE
          valueFrom:
            fieldRef:
              fieldPath: metadata.namespace
        - name: POD_IP
          valueFrom:
            fieldRef:
              fieldPath: status.podIP

xref GIE issue: kubernetes-sigs/gateway-api-inference-extension#1680

[danehans]

cc: @robscott