Add optional user_id to getAccessToken()

[jdelaune]

https://github.com/bshaffer/oauth2-server-php/blob/develop/src/OAuth2/Storage/AccessTokenInterface.php#L26

Would be good to add an optional user_id to the return on this method. I've implemented in the symfony bundle, but it would be good to add it to the comments here.

[bshaffer]

I added this to the comments. All the built-in storage engines already return the user_id as part of the getAccessToken response, so the only thing to really do here is update the doc block.