text: Title: Renderer should be configurable on a per-router or per-endpoint basis
  Body: **Is your feature request related to a problem? Please describe.**
  I am trying to export metrics of my application via `prometheus_client`. Prometheus will consume plain-text data from any HTTP endpoint and parse it.
  All my endpoints are JSON, except the metrics endpoint, which should be plaintext.
  
  **Describe the solution you'd like**
  I'd like to be able to, ideally, set my renderer on a per-endpoint basis:
  
  ```python
  @router.get("/", renderer=MyRenderer)
  def endpoint(request):
      ...
  ```
  
  Alternatively, I would be OK with having the router itself take a renderer:
  
  ```python
  router = Router(renderer=MyRenderer)
  ```
  
  What I have done now works, but seems like a hack, I bypass the renderer selection when bypassing `create_response` by returning an `HttpResponse`
  
  ```python
  @router.get("/", response=str)
  def endpoint(request):
      return HttpResponse("some plain text", status=200, content_type="text/plain")
  ```