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/legacy_test.go

@@ -60,3 +60,21 @@ func TestDialerLegacyAdapterDefaults(t *testing.T) {
 	r := NewDialerLegacyAdapter(&net.Dialer{})
 	r.CloseIdleConnections() // does not crash
 }
+
+func TestQUICContextDialerAdapterWithCompatibleType(t *testing.T) {
+	var called bool
+	d := NewQUICDialerFromContextDialerAdapter(&mocks.QUICDialer{
+		MockCloseIdleConnections: func() {
+			called = true
+		},
+	})
+	d.CloseIdleConnections()
+	if !called {
+		t.Fatal("not called")
+	}
+}
+
+func TestQUICContextDialerAdapterDefaults(t *testing.T) {
+	d := NewQUICDialerFromContextDialerAdapter(&mocks.QUICContextDialer{})
+	d.CloseIdleConnections() // does not crash
+}