feat: add terminal_storage_class fields to Autoclass message

The terminal_storage_class field allows users to set the coldest storage class for Autoclass enabled buckets. An object in an Autoclass bucket will eventually cool down to the terminal storage class if there is no access to the object.

PiperOrigin-RevId: 569316060

google/storage/v2/storage.proto

@@ -1811,6 +1811,15 @@ message Bucket {
     // to the bucket creation time.
     google.protobuf.Timestamp toggle_time = 2
         [(google.api.field_behavior) = OUTPUT_ONLY];
+
+    // An object in an Autoclass bucket will eventually cool down to the
+    // terminal storage class if there is no access to the object.
+    // The only valid values are NEARLINE and ARCHIVE.
+    optional string terminal_storage_class = 3;
+
+    // Latest instant at which the autoclass terminal storage class was updated.
+    optional google.protobuf.Timestamp terminal_storage_class_update_time = 4
+        [(google.api.field_behavior) = OUTPUT_ONLY];
   }
 
   // Immutable. The name of the bucket.