Support start api with tls

### Describe your idea/feature/enhancement

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

I wish sam local start-api would support https.

### Proposal

Add details of how to add this to the product.

The start-api subcommand starts up a Flask app to simulate api gateway generated events. Most clients of api gateway expect an https interface. Flask supports an ssl_context.

Allow users to provide a self signed key and cert to opt into an https supported mode of start-api. Omitting these would have the same effect as we have today, a non tls interface.

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)

No.

### Additional Details