Support spec v0.9.1

[michielbdejong]

https://gitter.im/solid/test-suite?at=62c31be79a314a3ec429310a

The plan is that the next version 0.9.1 will have the Insecure WebSOocket notifications deprecated and secure authenticated web socket notifications added.

See solid/specification@3602385 for the new spec text.

I had a look through https://github.com/CommunitySolidServer/CommunitySolidServer/search?q=websockets&type=issues but couldn't really find any relevant work there, except for CommunitySolidServer#1388 which might have some related code.

See solid-contrib/specification-tests#86 for some tests that might help us.