Export SAM template outputs

I noticed the generated SAM templates have some very useful output values, such as the EndpointURL. It would be really great if these were exported with a unique name, such as `"${AWS::StackName}-LogicalID"` so we could import them into other cloudformation templates (such as a frontend stack making requests to the Chalice api). Right now, it seems like the only way to get at them programmatically is to write a script using the aws command line tools to describe the stack and parse the result for outputs. A benefit of importing values directly into other CF templates is also that AWS will then restrict any actions that would change these exported values and break these other stacks.

Start by locating the code that generates SAM templates and inspect how values such as EndpointURL are represented in the generated CloudFormation outputs. Check the existing template-generation tests, if present, and verify that each output receives a unique export name that can be imported by another stack without breaking dependent-stack protections.

Written by the indexing model from the issue text.