[Feature Request] Add option to use app factory pattern

As chalice mimicks flask api,it would be useful to provide the possibility of the module defining a `create_app` method instead of an `app` module attribute.

My use case is connecting to an sqs queue on app creation and creating an attribute on the app for easier access

Start by tracing how Chalice discovers the module-level `app` entry point and compare that flow with the requested `create_app` entry point. Check how application attributes and SQS setup would be available after creation. Done means an app factory can be used without breaking existing app-attribute projects, with tests covering both patterns.

Written by the indexing model from the issue text.