feat(netxlite): implement hooks for transparent proxy (#561)

See https://github.com/ooni/probe/issues/1803#issuecomment-957323297.

internal/netxlite/tproxy.go

@@ -0,0 +1,59 @@
+package netxlite
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/netxlite/quicx"
+)
+
+// TProxable is the fundamental type used by the netxlite package to perform
+// low-level network operations for which, by default, we use the stdlib.
+//
+// The t stands for transparent. By using this type as the fundamental type,
+// we can transparently intercept connections and implement censorship
+// policies. The implementation of this functionality is not part of netxlite:
+// here we only have the basic mechanism to make this possible.
+type TProxable interface {
+	// ListenUDP creates a new quicx.UDPLikeConn conn.
+	ListenUDP(network string, laddr *net.UDPAddr) (quicx.UDPLikeConn, error)
+
+	// LookupHost lookups a domain using the stdlib resolver.
+	LookupHost(ctx context.Context, domain string) ([]string, error)
+
+	// NewTProxyDialer returns a new TProxyDialer.
+	NewTProxyDialer(timeout time.Duration) TProxyDialer
+}
+
+// TProxyDialer is the dialer type returned by TProxable.NewDialer.
+type TProxyDialer interface {
+	// DialContext behaves like net.Dialer.DialContext.
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// TProxy is the fundamental variable controlling how netxlite creates
+// net.Conn and quicx.UDPLikeConn, as well as how it uses the stdlib
+// resolver. By modifying this variable, you can effectively transparently
+// proxy netxlite (and hence OONI) activities to other services. This is
+// quite convenient when performing quality assurance tests.
+var TProxy TProxable = &TProxyStdlib{}
+
+// TProxyStdlib is the default TProxable implementation that uses
+// the stdlib in the most obvious way for every functionality.
+type TProxyStdlib struct{}
+
+// ListenUDP calls net.ListenUDP.
+func (*TProxyStdlib) ListenUDP(network string, laddr *net.UDPAddr) (quicx.UDPLikeConn, error) {
+	return net.ListenUDP(network, laddr)
+}
+
+// LookupHost calls net.DefaultResolver.LookupHost.
+func (*TProxyStdlib) LookupHost(ctx context.Context, domain string) ([]string, error) {
+	return net.DefaultResolver.LookupHost(ctx, domain)
+}
+
+// NewTProxyDialer returns a &net.Dialer{Timeout: timeout} instance.
+func (*TProxyStdlib) NewTProxyDialer(timeout time.Duration) TProxyDialer {
+	return &net.Dialer{Timeout: timeout}
+}