refactor(netxlite): better integration with tracex (#774)

Rather than passing functions to construct complex objects such
as Dialer and QUICDialer, pass interface implementations.

Ensure that a nil implementation does not cause harm.

Make Saver implement the correct interface either directly or
indirectly. We need to implement the correct interface indirectly
for TCP conns (or connected UDP sockets) because we have two
distinct use cases inside netx: observing just the connect event
and observing just the I/O events.

With this change, the construction of composed Dialers and
QUICDialers is greatly simplified and more obvious.

Part of https://github.com/ooni/probe/issues/2121

internal/engine/netx/dialer/dialer.go

@@ -54,21 +54,10 @@ func New(config *Config, resolver model.Resolver) model.Dialer {
 	if config.Logger != nil {
 		logger = config.Logger
 	}
-	modifiers := []netxlite.DialerWrapper{
-		func(dialer model.Dialer) model.Dialer {
-			if config.DialSaver != nil {
-				dialer = &tracex.SaverDialer{Dialer: dialer, Saver: config.DialSaver}
-			}
-			return dialer
-		},
-		func(dialer model.Dialer) model.Dialer {
-			if config.ReadWriteSaver != nil {
-				dialer = &tracex.SaverConnDialer{Dialer: dialer, Saver: config.ReadWriteSaver}
-			}
-			return dialer
-		},
-	}
-	d := netxlite.NewDialerWithResolver(logger, resolver, modifiers...)
+	d := netxlite.NewDialerWithResolver(
+		logger, resolver, config.DialSaver.NewConnectObserver(),
+		config.ReadWriteSaver.NewReadWriteObserver(),
+	)
 	d = &netxlite.MaybeProxyDialer{ProxyURL: config.ProxyURL, Dialer: d}
 	if config.ContextByteCounting {
 		d = &bytecounter.ContextAwareDialer{Dialer: d}