Add method to retrieve raw id_token from AccessToken object

[JoshuaLicense]

It would be convenient to be able to retrieve the id_token raw string directly from the AccessToken object (consistent with other methods: getToken() & getRefreshToken()).

Before:

$token->getValues()['id_token'];

After:

$token->getIdToken();

Note: afe3b06 (unsetting from $this->values) is a breaking change - I'm happy to revert this if needed.