Allow pass-through of ECS from client to upstream server

In some cases clients send DNS requests with ECS (e.g. dnsmasq behind dnsproxy can send ECS via `--add-subnet`). But it does not seem to be passed upstream by dnsproxy, or at least I can't get it to work.

Can you please add a config option `--edns-pass-to-upstream`? The logic should probably be: if client supplies it, pass it through. If the client doesn't supply it and `--edns` or `--ends-addr` are enabled, use those.

It can be tested like this:
`dig o-o.myaddr.google.com txt +subnet='1.2.3.0/24' @8.8.8.8` (replace 8.8.8.8 with the address of dnsproxy)