feature: Allow selecting inference response content for automl generated models (aws#1566)

*  Added attach() method to AutoML to allow attaching an AutoML object
to an existing AutoMLJob
*  Added create_model() method to AutoML that returns a PipelineModel
that can be used by the
*  Added a util function validate_and_update_inference_response() to
update the inference container list based on the requested
inference response keys
* Updated and added unit tests

Author: pdasamzn