Add auth_fief plugin, register auth via entry point, add WebSocket auth

[davidbrochart]

This PR makes it possible to swap auth. Previously, there was a hard dependency on fps-auth, which is the "embedded" auth (doesn't require a separate server, has the database, registers auth endpoints in jupyverse). Now any auth can be installed, and it must register itself in an entry point jupyverse_auth group, with the following entry points: User, current_user, update_user and websocket_auth.
fps-auth-fief is an auth that uses https://www.fief.dev, which is separate from jupyverse and can run in the cloud or locally. Users must authenticate in Fief before being able to access e.g. JupyterLab, and depending on the permissions they are granted, they can use different functionalities (read or write files, execute notebooks, etc.).

fief.mp4

[davidbrochart]

Commit Add WebSocket authorization has the changes of #210.