Update stage name for REST Gateways to use $default

[gordon-klotho]

https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-stages.html

You can create a $default stage that is served from the base of your API's URL—for example, https://{api_id}.execute-api.{region}.amazonaws.com/. You use this URL to invoke an API stage.

This would enable customers to use plain CNAME for DNS instead of requiring Custom Domain Mapping and just make it generally easier to interact with.

Backwards-incompatible

This would break existing deployments' custom domain mapping, so we need to be careful about timing and messaging.

[atorres-klo]

This will require changes to how we wire cloud front as well

[jhsinger-klotho]

i dont think the default stage name works with REST apis, the doc linked is purely for http. I wasnt able to figure it out at least and saw

    	* creating API Gateway Stage ($default): BadRequestException: Stage name only allows a-zA-Z0-9_
    	* ```

[gordon-klotho]

Ahh, yeah, it appears to be a v2 only feature