Support hook_openid_client interaction when user is adding openid identities

When user is logging in modules which have implemented hook_openid_ax_client could inject theirs own properties using op "get" and save these properties using op "save".

However when user is logged in and trying to add openid identities through the Openid Identities tab under its account, only "get" op is going to be fired - but injected properties couldn't be processed and saved.

In the patch I introduced new op "add" which is launched after user successfully adds new openid identity.

Desired example usage:

/**
 * Implementation of hook_openid_client()
 */
function mymodule_openid_client($op, $response = NULL, $account = NULL) {
  switch ($op) {
    case 'get':
      $mapping = _mymodule_mappings();
      return $mapping;
    case 'save':
    case 'add':
      _mymodule_openid_client_save($response, $account);
  }
}

Note: The patch could be applied after the patch #1033292-3: Support AX extensions + namespace collision bug.