Add ability to batch Storage.create() requests #2351

Seems like current batching API [1] does not support batching create [2] requests. Batching create requests might be needed for some use cases; for example, if a user needs to create a large number of small files.

If this is already supported it might be good to add an example.

[1] https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-storage/src/main/java/com/google/cloud/storage/StorageBatch.java#L55
[2] https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-storage/src/main/java/com/google/cloud/storage/Storage.java#L1427