Add support for event sources configured in the same project

I have a dynamodb created in a CF template that I have chalice merge with it's generated stuff, but I can't reference that in the chalice `@app.on_dynamo_record` because it requires an already created stream arn. it would be nice to be able to reference something in a merged CF template.

Start with the @app.on_dynamo_record entry point and the chalice merge behavior described in the issue, then trace how the generated CloudFormation template handles DynamoDB stream ARNs. Done means an event source can reference a DynamoDB resource defined in the merged template without requiring a pre-existing stream ARN.

Written by the indexing model from the issue text.