[Enhancement] SageMaker async inference

[athewsey]

To host the model, this sample currently deploys a real-time SageMaker endpoint backed by GPU - which may be fine for high-volume use cases but probably pretty resource-intensive for many.

Since the end-to-end workflow here is asynchronous anyway (may have a human review component), it's probably a good case for new SageMaker asynchronous inference feature which supports scaling down to zero when demand is low.

• Wait until async inference is supported in SageMaker Python SDK, rather than confusing the notebook with boto3 endpoint setup (tracking their issue and pull request)
• Update notebook 2 to create an async endpoint with scale-to-zero capability
• Update pipeline to correctly consume async endpoints

TBD: Do we need to retain a real-time deployment option for anybody that wants to optimize for low latency? Seems unnecessary to me at the moment