Add `accepted_renderer` as an official attribute in `rest_framework.request.Request`

[Tomer Bu]

Currently, accepted_renderer is dynamically added to the Request object during APIView.initialize_request(), but it is not explicitly defined in the Request class.

This makes it hard to know it exists, unless developers dig into the source code or check an APIView request at runtime.

please add a default value of None before content negotiation.