Add streaming interface for pubsub publishers

[lthibault]

It is common for pubsub.Topic.Publish() to be called repeatedly in order to stream events to a topic. However, this method blocks until the RPC call round-trips, which degrades performance. While we currently expose PublishAsync, the proper management of stream state is a non-trivial task, so we should expose a NewStream() method that integrates with CASM's pkg/util/stream.

It may be worthwhile to reuse code from #59.