Eliminate unnecessary login processing on every LTI launch

Currently LTIToolProviderBase::authenticate calls userLoginFinalize, which in turn just calls drupal's user_login_finalize(). This happens on every launch, which means it happens whenever a user views a resource via LTI. user_login_finalize(), however can potentially do quite a lot, as it starts up a new user session, updates the user's last login time, fires off events, and invokes all user_login hooks. This is unnecessary, if the same user is viewing different resources within a sitting.

Unnecessary processing could be avoided, if the login finalize process could be conditioned upon the LTI user's 'sub' value changing.

To illustrate this, see the following code:

 /**
   * Check if the user is a new login.
   *
   * @param $session
   *   The session.
   *
   * @return bool
   */
  public function isNewLogin($session) {
    $context_sub = $session->get('lti_tool_provider_context')->getUserIdentity()->getIdentifier();
    $session_sub = $session->get('lti_tool_provider_context_sub');
    return $context_sub !== $session_sub;
  }

  /**
   * {@inheritdoc}
   */
  public function userLoginFinalize(UserInterface $user) {
    user_login_finalize($user);
    $session = \Drupal::service('session');
    $session->set('lti_tool_provider_context_sub', $session->get('lti_tool_provider_context')->getUserIdentity()->getIdentifier());
  }

One additional thing to note: Drupal's user_login_finalize is about to be deprecated in 10.3: https://www.drupal.org/node/3379194 →