Enhancement : add id_token generation example

[mati-o]

I believe it is critical to include an example of JWT id_token generation since OIDC id_tokens are all about JWT as far as I get it from the specs. So it would be nice to get an example of best practices and recommended modules to use for generating a JWT id_token.

Currently, all OIDC extensions in the example leave the id_token generation up to the user. Which to my opinion is confusing

server.grant(oauth2orize_ext.grant.idToken(function(client, user, done){
  var id_token;
  // Do your lookup/token generation.
  // ... id_token =

  done(null, id_token);
}));

https://github.com/gerges-beshay/oauth2orize-openid-examples/blob/master/oauth2.js#L43