fix(sessionresolver): honour the proxy (#250)

In reality, we are not going to use the sessionresolver when we're
using a proxy (I just tested). But, it nonetheless feels a lot more
robust to write a correct sessionresolver that handles the proxy
in the most correct way. That is, the sessionresolver will now skip
all the entries that cannot use a socks5 proxy (including among them
also the system resolver). What's more, it will construct a child
resolver that propagates the proxy.

We have confidence that this holds true because we have added a test
ensuring that we are really using the configured proxy.

See https://github.com/ooni/probe/issues/1381

internal/engine/internal/sessionresolver/resolvermaker.go

@@ -88,6 +88,7 @@ func (r *Resolver) newresolver(URL string) (childResolver, error) {
 		ByteCounter:  r.byteCounter(),
 		HTTP3Enabled: h3,
 		Logger:       r.logger(),
+		ProxyURL:     r.ProxyURL,
 	}, URL)
 }