Implement linter to warn for required fields in `body`

We are exploring the use of required fields in request/response bodies. Often, these can be very problematic, e.g. if something changes from optional to required, you want to return/send only a 'partial' resource, etc. Because of versioning, client and server code has to handle changes to the resource structure anyway. Making everything optional enables greater flexibility in what can flow over the wire.

> Mark a field as `required` only when it must absolutely be present on every request/response for every version of the API.