Add support for UserInfo OIDC endpoints

[realjoshparker]

Using an Idp like Authelia it doesn't share the preferred_username in the normal token request and must be obtained through a subsequent call to the UserInfo endpoint by default. This isn't a major issue in and of itself, but when you're auto-creating users it results in getting the sub as the username which is not ideal as this needs to be manually updated by an admin to something more human readable.

Ideally we would implement an optional request to the UserInfo endpoint when creating users to obtain the preferred_username when creating the account.

As a side note this is fairly easy to work around using the claims_policies to add additional fields to the token request. But this is something the Authelia team highly discourages.

...
  identity_providers:
    oidc:
      claims_policies:
        legacy-for-cc:
          id_token: [ email, email_verified, name, preferred_username ]
    clients:
      - client_name: 'Christmas Community'
        claims_policy: legacy-for-cc
        ...