feat: Adding unreachable field to ListDataSourceReferences API (#32162)

* feat: Adding `unreachable` field to `ListDataSourceReferences` API

PiperOrigin-RevId: 828740816

Source-Link: googleapis/googleapis@73c0641

Source-Link: googleapis/googleapis-gen@ba4d457
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google-cloud-backupdr-v1/lib/google/cloud/backupdr/v1/datasourcereference_pb.rb

@@ -151,6 +151,9 @@ class ListDataSourceReferencesRequest
         #   @return [::String]
         #     A token, which can be sent as `page_token` to retrieve the next page.
         #     If this field is omitted, there are no subsequent pages.
+        # @!attribute [rw] unreachable
+        #   @return [::Array<::String>]
+        #     Locations that could not be reached.
         class ListDataSourceReferencesResponse
           include ::Google::Protobuf::MessageExts
           extend ::Google::Protobuf::MessageExts::ClassMethods