Support for specifying stages in pipeline.json file [feature-request]

Hi,
It would be really nice if there is a way to specify stages for Chalice APis when the chalice --generate-pipeline command is triggered.

Start with the chalice --generate-pipeline command and the pipeline.json file mentioned in the request. Determine how stages should be specified for Chalice APIs and define the generated pipeline behavior; done means pipeline.json can express stages and --generate-pipeline produces the expected staged pipeline.

Written by the indexing model from the issue text.