Allow student LTI launches to be anonymous on Drupal

Currently, the module assumes that all LTI users will be set up as accounts on drupal.

A nice feature would be to exclude certain LTI roles from creating accounts or being logged in to Drupal.

This could be fairly easy to do in \Drupal\lti_tool_provider\Authentication\Provider\LTIToolProviderBase::authenticate method. If the session's role value includes a certain value or values for the role (which could come from a configuration), then bypass the normal provisioning, etc., and set the session. If the 'sub' value is different for the user than the current session value for the sub (see related issue to this one on how to do this), then run userLoginFinalize on User::getAnonymousUser().