Allow "sub" as second option (to "id") for the ressource owner user id

[Jefferson49]

The current implementation of the generic oauth2-client, expects the ressource owner to have an "id":
https://github.com/thephpleague/oauth2-client/blob/master/src/Provider/GenericProvider.php#L79

If the response with the user data from the authorization server does not have an "id", the following code will throw a PHP error:
https://github.com/thephpleague/oauth2-client/blob/master/src/Provider/GenericResourceOwner.php#L49

However, many OpenID Connect implementations, return "sub" as user ID. Since the "sub" claim is the typical unique identifier for the end-user in Open ID Connect, I propose to allow "sub" as a second option in function getId(); getId could check for "id" first, and check "sub" in a second step.