Provide a way to use a Forward Proxy with an Egress

When a kubernetes cluster is behind a forward proxy, mesh egress pods within the cluster have no ability to further proxy the requests for ExternalServices through that forward proxy.

Curl has the -x option which tunnels through a proxy. We would like a similar option to enable an Egress to do the same thing.

I have tried setting https_proxy and http_proxy environment variables in the Egress pod but these variables are not honored currently.

The workaround is to create an ExternalService that points to the forward proxy, then change the application code accordingly to tunnel through the forward proxy ExternalService.