chore: Add protobuf-util-java dep

Author: lqiu96

java-datastore/google-cloud-datastore/pom.xml

@@ -79,6 +79,10 @@
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java</artifactId>
     </dependency>
+    <dependency>
+      <groupId>com.google.protobuf</groupId>
+      <artifactId>protobuf-java-util</artifactId>
+    </dependency>
     <dependency>
       <groupId>com.google.api</groupId>
       <artifactId>gax</artifactId>