Add "Sign in with Apple" support

[erral]

As explained in #46 ...

Sign in with Apple is an OpenID Connectish interface to allow users sign in using their Apple credentials.

It has some peculiarities like:

• It uses the client_secret_post authentication method when requesting the access token
• It needs to create a specific secret based on the private key instead of a fixed client_secret, which in itself is a JWT token but it doesn't use the private_key_jwt authentication mechanism that OIDC provides 😥
• It returns user's name only after first login and as a response to the access token request and not in the user info phase 🙈

I am not sure whether introducing Apple weirdness in this plugin is OK, but as it mostly complies with OIDC I have added a PR. And also because I am running in on a small production site.

I am open to discuss it.

[erral]

@ericof can you help me with the dependencies check?

I have introduced PyJWT as a dependency but the checker says I am missing the jwt dependency. The point is that when installing PyJWT you have to do import jwt. Yes. I know. One more in the wonderful python packaging problem (and this is better than djangorestframework-jwt).

I tried adding it to .meta.toml as suggested in the pyproject.toml but I don't get how to do the mapping between them,