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/model/mocks/underlyingnetwork.go

@@ -0,0 +1,38 @@
+package mocks
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+// UnderlyingNetwork allows mocking model.UnderlyingNetwork.
+type UnderlyingNetwork struct {
+	MockDialContext func(ctx context.Context, timeout time.Duration, network, address string) (net.Conn, error)
+
+	MockListenUDP func(network string, addr *net.UDPAddr) (model.UDPLikeConn, error)
+
+	MockGetaddrinfoLookupANY func(ctx context.Context, domain string) ([]string, string, error)
+
+	MockGetaddrinfoResolverNetwork func() string
+}
+
+var _ model.UnderlyingNetwork = &UnderlyingNetwork{}
+
+func (un *UnderlyingNetwork) DialContext(ctx context.Context, timeout time.Duration, network, address string) (net.Conn, error) {
+	return un.MockDialContext(ctx, timeout, network, address)
+}
+
+func (un *UnderlyingNetwork) ListenUDP(network string, addr *net.UDPAddr) (model.UDPLikeConn, error) {
+	return un.MockListenUDP(network, addr)
+}
+
+func (un *UnderlyingNetwork) GetaddrinfoLookupANY(ctx context.Context, domain string) ([]string, string, error) {
+	return un.MockGetaddrinfoLookupANY(ctx, domain)
+}
+
+func (un *UnderlyingNetwork) GetaddrinfoResolverNetwork() string {
+	return un.MockGetaddrinfoResolverNetwork()
+}