Enable inverse proxy to access host network (#2614)

When switching to GKE workload identity, the pods can't access to metadata server anymore by default due to metadata concealment. 
This can be unlocked by explicitly enable hostnetwork for the pod. 
https://cloud.google.com/kubernetes-engine/docs/how-to/protecting-cluster-metadata#concealment

This should be OK as proxy is an optional component. In any case when user feel this not a secure option he/she could opt out it.

manifests/kustomize/base/proxy/proxy-deployment.yaml

@@ -13,8 +13,9 @@ spec:
       labels:
         app: proxy-agent
     spec:
+      hostNetwork: true
       containers:
       - image: gcr.io/ml-pipeline/inverse-proxy-agent:0.1.20
         imagePullPolicy: IfNotPresent
         name: proxy-agent
-      serviceAccountName: proxy-agent-runner
+      serviceAccountName: proxy-agent-runner