Support named examples (a la OpenAPI 3) on parameters, request bodies, responses, models, and model properties

[mikekistler]

We should have a mechanism to provide examples right in the Cadl file for individual parameters / model properties.

In OpenAPI 3 there is the ability to specify named examples on parameters, request bodies, and response bodies. So all example values with a common name, like "Creating a yellow widget", can be viewed as an example of a request and the expected response to that request. This is useful for documentation but also for test generation.