[alts]: store tsi peer property in dynamic metadata. (#17196)

Signed-off-by: chaoqin-li1123 <chaoqinli@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 20abecf04044d02714f07ed2aa5cf699983c08ca

envoy/extensions/transport_sockets/alts/v3/alts.proto

@@ -15,6 +15,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // [#extension: envoy.transport_sockets.alts]
 
 // Configuration for ALTS transport socket. This provides Google's ALTS protocol to Envoy.
+// Store the peer identity in dynamic metadata, namespace is "envoy.transport_socket.peer_information", key is "peer_identity".
 // https://cloud.google.com/security/encryption-in-transit/application-layer-transport-security/
 message Alts {
   option (udpa.annotations.versioning).previous_message_type =