[Feature request] Package a chalice app as a Lambda layer

Sometimes there are python functions that are used across multiple lambda functions. These are usually helper functions or additional dependencies to be injected or just a common set of `requirements.txt` to have a consistent environment.

It would be useful if we can package a chalice app as a layer and use that layer as a dependency like a standard python module in other lambda functions.

Start by examining how Chalice currently packages an app and how AWS Lambda layers are used as dependencies. Define the packaging workflow and integration expectations needed for a Chalice app to become a reusable layer across multiple Lambda functions; the issue does not name files or tests.

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