refactor: introduce factory for stdlib http transport (#413)

With this factory, we want to construct ourselves the TLS dialer
so that we can use a dialer wrapper that always sets timeouts when
reading, addressing https://github.com/ooni/probe/issues/1609.

As a result, we cannot immediately replace the i/e/netx factory
for creating a new HTTP transport, since the functions signatures
are not directly compatible.

Refactoring is part of https://github.com/ooni/probe/issues/1505.

internal/netxlite/http3.go

@@ -47,8 +47,13 @@ func NewHTTP3Transport(
 	dialer QUICContextDialer, tlsConfig *tls.Config) HTTPTransport {
 	return &http3Transport{
 		child: &http3.RoundTripper{
-			Dial:            (&http3Dialer{dialer}).dial,
-			TLSClientConfig: tlsConfig,
+			Dial: (&http3Dialer{dialer}).dial,
+			// The following (1) reduces the number of headers that Go will
+			// automatically send for us and (2) ensures that we always receive
+			// back the true headers, such as Content-Length. This change is
+			// functional to OONI's goal of observing the network.
+			DisableCompression: true,
+			TLSClientConfig:    tlsConfig,
 		},
 	}
 }