text: Title: Add option for `Access-Control-Allow-Private-Network` to CORSMiddleware
  Body: Add option to enable [`Access-Control-Allow-Private-Network`](https://wicg.github.io/private-network-access/#http-headerdef-access-control-request-private-network) to CORSMiddleware.
  
  We probably don't need to inject it unconditionally, but only in response to the `Access-Control-Request-Private-Network` request header (when set to to `true`).
  
  The option **must be disabled** by default.