Allow a "no runtime" into `sam init`

### Describe your idea/feature/enhancement
Our app template repo had a contribution for allowing apps with no runtime defined (https://github.com/aws/aws-sam-cli-app-templates/pull/48). This is a great use-case for apps that may not have a Lambda Function within, maybe a sole step function app.

Provide a clear description. Ex. I wish SAM CLI would [...]

### Proposal

In `sam init` we would allow a "no runtime" option that allowed us to build these kinds of start templates into our init experience.

Things to consider:
1. Will require app templates updates.

### Additional Details

Note the to community. If this is something you want, please 👍. If someone is interested in solving this, please let us know so we can work through designs and requirements with you.