Add support for Cognito event sources

SAM provides ability to add triggers for Cognito events. Documentation here:
https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-property-function-cognito.html

Currently Chalice does not support Cognito as an event source:
https://aws.github.io/chalice/topics/events.html

Other than creating a custom resource lambda, I'm not sure if there's a way to attach a lambda function created in chalice to a Cognito event. It would be great if we can just configure Cognito events from within Chalice.

Please close if this functionality already exists.

Start with Chalice's events documentation and the linked AWS SAM Cognito event-source documentation to identify the Cognito event types and configuration that need support. Determine how a Chalice-created Lambda can be attached without a custom resource, then verify that the supported configuration creates the expected Cognito event wiring.

Written by the indexing model from the issue text.