Remove cross-pod unix domain sockets

In Istio versions before 1.5, during secret discovery service (SDS) execution, the SDS client and the SDS server communicate through a cross-pod Unix domain socket (UDS), which needs to be protected by Kubernetes pod security policies.

With Istio 1.5, Pilot Agent, Envoy, and Citadel Agent will be running in the same container (the architecture is shown in the following diagram). To defend against attackers eavesdropping on the cross-pod UDS between Envoy (SDS client) and Citadel Agent (SDS server), Istio 1.5 merges Pilot Agent and Citadel Agent into a single Istio Agent and makes the UDS between Envoy and Citadel Agent private to the Istio Agent container. The Istio Agent container is deployed as the sidecar of the application service container.