feat(netxlite): implement LookupHTTPS (#514)

This new API call performs DNS lookups for HTTPS records.

Part of https://github.com/ooni/probe/issues/1733 and diff has been
extracted from https://github.com/ooni/probe-cli/pull/506.

internal/netxlite/legacy_test.go

@@ -1,6 +1,8 @@
 package netxlite
 
 import (
+	"context"
+	"errors"
 	"net"
 	"testing"
 
@@ -43,6 +45,17 @@ func TestResolverLegacyAdapter(t *testing.T) {
 		}
 		r.CloseIdleConnections() // does not crash
 	})
+
+	t.Run("for LookupHTTPS", func(t *testing.T) {
+		r := NewResolverLegacyAdapter(&net.Resolver{})
+		https, err := r.LookupHTTPS(context.Background(), "x.org")
+		if !errors.Is(err, ErrNoDNSTransport) {
+			t.Fatal("not the error we expected")
+		}
+		if https != nil {
+			t.Fatal("expected nil result")
+		}
+	})
 }
 
 func TestDialerLegacyAdapter(t *testing.T) {