Add "Sign in with Apple" extra configuration

[erral]

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 🙈

Together with these changes I have added the support for adding extra OIDC plugins (in case you need to have more than one OIDC providers in your site).

[erral]

I don't know whether this is the place to implement Apple specifics or it is better to have a pas.plugins.appleid to do so...

[erral]

I will split this PR in 2:

• AppleId support
• Generalize plugin and endpoint to support several plugin configuration in a single Plone instance, allowing adding more than one plugin with different Ids.