Optionally add template outputs to Parameter Store

[benkehoe]

This app can be deployed once to an account and used by multiple state machines. Any code that uses the app shouldn't have to hardcode the Lambda function name or API Gateway URL. So there should be a way to store those values to let that code look up the values for a centrally-deployed instance of this app.

This must be opt-in. If someone is using the SAR app embedded in their own CloudFormation template, the app should stay "local" to that stack, as there may be many instances of the app deployed and they should not all attempt to write their outputs to that central place.

Parameter Store is a good option for this; the lookup is then possible for code deployed directly or using CloudFormation. It would be nice to use CloudFormation exports as well, but these cannot be made conditional at this time.