fix(bigquery): declare closed as transient volatile in BigQueryImpl

Author: jinseopkim0

java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/BigQueryImpl.java

@@ -504,7 +504,7 @@ private void closeClient() {
 
   private transient ConcurrentHashMap<String, BigQueryReadClient> bqReadClients;
   private transient boolean isGlobalClientUserProvided;
-  private boolean closed = false;
+  private transient volatile boolean closed = false;
 
   /**
    * Lazily creates or retrieves the shared {@link BigQueryReadClient} instance used for streaming