feat(netxlite): add CloseIdleConnections to quic dialer (#469)

Like before, do not touch the rest of the tree. Rather create
compatibility types declared as legacy.

We will soon be able to close idle connections for an HTTP3
transport using any kind of resolvers more easily.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/mocks/legacy_test.go

@@ -0,0 +1,29 @@
+package mocks
+
+import (
+	"context"
+	"crypto/tls"
+	"errors"
+	"testing"
+
+	"github.com/lucas-clemente/quic-go"
+)
+
+func TestQUICContextDialerDialContext(t *testing.T) {
+	expected := errors.New("mocked error")
+	qcd := &QUICContextDialer{
+		MockDialContext: func(ctx context.Context, network string, address string, tlsConfig *tls.Config, quicConfig *quic.Config) (quic.EarlySession, error) {
+			return nil, expected
+		},
+	}
+	ctx := context.Background()
+	tlsConfig := &tls.Config{}
+	quicConfig := &quic.Config{}
+	sess, err := qcd.DialContext(ctx, "udp", "dns.google:443", tlsConfig, quicConfig)
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if sess != nil {
+		t.Fatal("expected nil session")
+	}
+}