Replace OAuth2 client by the one from thephpleague

Issue #2879665 on drupal.org by pcambra.

I had some issues with the client credentials OAuth2 plugin of the client currently in use, I think a much more maintained and supported option is https://github.com/thephpleague/oauth2-client. The server version of this package is being used in https://www.drupal.org/project/simple_oauth v2.