HOTFIX: Fix optional import in ConsumerCoordinator (#6953)

This was caused by back-to-back merging of #6854 (which removed the Optional import) and #6936 (which needed the import).

Reviewers: Jason Gustafson <jason@confluent.io>

clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerCoordinator.java

@@ -64,6 +64,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.atomic.AtomicBoolean;