Port https://github.com/grpc/grpc-go/pull/6686 to C++ (#37950)

This fixes connecting to a local ATLS server with grpc_cli.

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Closes #37950

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37950 from rainwoodman:patch-1 1172618fb2
PiperOrigin-RevId: 687975188

src/core/lib/security/credentials/alts/alts_credentials.cc

@@ -29,7 +29,7 @@
 #include "src/core/lib/security/credentials/alts/grpc_alts_credentials_options.h"
 #include "src/core/lib/security/security_connector/alts/alts_security_connector.h"
 
-#define GRPC_ALTS_HANDSHAKER_SERVICE_URL "metadata.google.internal.:8080"
+#define GRPC_ALTS_HANDSHAKER_SERVICE_URL "dns:///metadata.google.internal.:8080"
 
 grpc_alts_credentials::grpc_alts_credentials(
     const grpc_alts_credentials_options* options,