This is a pretty basic question, sorry – no kubernetes experience before using Istio.
Can you use environment variables like $VAR_NAME inside your config for Istio items like VirtualService etc.? e.g. as mentioned here: https://kubernetes.io/docs/tasks/inject-data-application/define-environment-variable-container/#using-environment-variables-inside-of-your-config
Is that what the $PROXY_IP and $PROXY_PORT are in this example? https://istio.io/latest/docs/tasks/traffic-management/egress/http-proxy/#configure-traffic-to-external-https-proxy