Support PubSub on AppEngine Standard

Pubsub doens't work on GAE standard. Please add support for GAEStandard.
pubsub-java throws the following error on initialization
with

      PubSub pubsub = PubSubOptions.getDefaultInstance().getService();

  error:
   java.lang.NoClassDefFoundError: java.nio.channels.spi.SelectorProvider is a restricted class.
 Please see the Google App Engine developer's guide for more details. at

(i suspect its not supported if the library now is gRPC only)

This is documented in our main README. The reason this is not supported on AppEngine is in fact gRPC not working AE, there is work on the AE side to fix this.

The other way to look at this is if gcloud-java's pubsub library would support REST (like the datastore implementation or python).
Is the suggestion to wait for AE to support gRPC (vs this client to support rest)?