After deploying a SAM application, check if the stack has an API gateway and print its URL

Feedback from @normj:

> Nice to have would be in the console after successfully deploying the application look to see if the CloudFormation stack has an API Gateway and if so print out the URL to it. Otherwise a new user after a deploying the initial application won't know where to go to see what they deployed.

No files or tests are named. Start by tracing the successful SAM application deployment flow and its console output, then identify how to detect an API Gateway in the deployed CloudFormation stack and expose its URL; done means the URL is printed after deployment when one exists, without disrupting deployments without one.

Written by the indexing model from the issue text.