Support DNS resolution via SOCKS5 proxy

[bison]

Support for connecting via a SOCKS5 proxy was added in #474 (thanks for that!) but it looks like DNS resolution still happens on the local end. It would be great if we could have DNS resolution also happen via the proxy. It's common that the DNS only exists in the internal network.

[tstirrat15]

It looks like you can set dns:/// as a target, which may be related: https://grpc.io/docs/guides/custom-name-resolution/#overview

But I'll need to do a bit more digging to figure out what's needed here.

[bison]

From the WithContextDialer docs:

Note that gRPC by default performs name resolution on the target passed to NewClient. To bypass name resolution and cause the target string to be passed directly to the dialer here instead, use the "passthrough" resolver by specifying it in the target string, e.g. "passthrough:target".

Haven't tested that, but it sounds like the right thing.