feat: reintroduce the tproxy functionality (#975)

We originally removed the TProxy in https://github.com/ooni/probe/issues/2224. Nevertheless, in https://github.com/ooni/probe-cli/pull/969, we determined that something like the previous TProxy, with small changes, was required to support https://github.com/ooni/probe/issues/2340. So, this pull request reintroduces a slightly-modified TProxy functionality that better adapts to the `--remote=REMOTE` use case.

internal/netxlite/tproxy.go

@@ -0,0 +1,39 @@
+package netxlite
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// TProxy refers to the UnderlyingNetwork implementation. By overriding this
+// variable you can force netxlite to use alternative network primitives.
+var TProxy model.UnderlyingNetwork = &DefaultTProxy{}
+
+// defaultTProxy is the default UnderlyingNetwork implementation.
+type DefaultTProxy struct{}
+
+// DialContext implements UnderlyingNetwork.
+func (tp *DefaultTProxy) DialContext(ctx context.Context, timeout time.Duration, network, address string) (net.Conn, error) {
+	d := &net.Dialer{
+		Timeout: timeout,
+	}
+	return d.DialContext(ctx, network, address)
+}
+
+// ListenUDP implements UnderlyingNetwork.
+func (tp *DefaultTProxy) ListenUDP(network string, addr *net.UDPAddr) (model.UDPLikeConn, error) {
+	return net.ListenUDP(network, addr)
+}
+
+// GetaddrinfoLookupANY implements UnderlyingNetwork.
+func (tp *DefaultTProxy) GetaddrinfoLookupANY(ctx context.Context, domain string) ([]string, string, error) {
+	return getaddrinfoLookupANY(ctx, domain)
+}
+
+// GetaddrinfoResolverNetwork implements UnderlyingNetwork.
+func (tp *DefaultTProxy) GetaddrinfoResolverNetwork() string {
+	return getaddrinfoResolverNetwork()
+}