chore(spanner): add overriden method for readwrite txn in multiplexed session

Author: harshachinta

google-cloud-spanner/src/main/java/com/google/cloud/spanner/MultiplexedSessionDatabaseClient.java

@@ -21,6 +21,7 @@
 import com.google.api.core.ApiFuture;
 import com.google.api.core.ApiFutures;
 import com.google.api.core.SettableApiFuture;
+import com.google.cloud.spanner.Options.TransactionOption;
 import com.google.cloud.spanner.SessionClient.SessionConsumer;
 import com.google.cloud.spanner.SpannerException.ResourceNotFoundException;
 import com.google.common.annotations.VisibleForTesting;
@@ -388,6 +389,11 @@ public ReadOnlyTransaction readOnlyTransaction(TimestampBound bound) {
     return createMultiplexedSessionTransaction(false).readOnlyTransaction(bound);
   }
 
+  @Override
+  public TransactionRunner readWriteTransaction(TransactionOption... options) {
+    return createMultiplexedSessionTransaction(false).readWriteTransaction(options);
+  }
+
   /**
    * It is enough with one executor to maintain the multiplexed sessions in all the clients, as they
    * do not need to be updated often, and the maintenance task is light. The core pool size is set