make DatastoreBatchWriter public (#3387)

It would be useful for `DatastoreBatchWriter` to be public for instrumentation purposes. Its sibling interfaces `DatastoreWriter`, `DatastoreReaderWriter`, `DatastoreReader` etc are all public so I assume that it not being public is not intentional.

Author: danielnorberg

google-cloud-clients/google-cloud-datastore/src/main/java/com/google/cloud/datastore/DatastoreBatchWriter.java

@@ -22,7 +22,7 @@
  * An interface to represent a batch of write operations. All write operation for a batch writer
  * will be applied to the Datastore in one RPC call.
  */
-interface DatastoreBatchWriter extends DatastoreWriter {
+public interface DatastoreBatchWriter extends DatastoreWriter {
 
   /**
    * Datastore add operation. This method will also allocate id for any entity with an incomplete