JWK sets

[einrobin]

Addresses #734

Still a draft since I wanna do some more testing myself, add unit tests and also write some documentation on it.

The config looks something like this:

    [auth]
    jwks_uri = "https://my-website/keys.json"
    jwks_refresh_interval = 3600 # comment out to disable continous refreshing
    # key = "/app/public.jwk"
    public = "anon"