gzip: Be able to only compress external responses

*gzip*: *Be able to only compress external responses*

*Description*: Since gzip is pretty computationally heavy, it would be great if envoy had the option to only gzip responses that will be external. We don't want to gzip at the front proxy layer, as our front proxy is CPU sensitive. But it would be great if we can compress just one hop over at the sidecar of edge services.

Ideally, this means if a request has the header `x-envoy-external-address`, the sidecar will compress the response.