Create `getIdentity` controller plugin

Per [the mailing list](https://groups.google.com/a/zend.com/d/msgid/apigility-users/c9268d98-122e-42c0-96da-b9e75c1b8450%40zend.com), we should expose a `getIdentity()` controller plugin to mimic the API in REST resources.

```php
public function getIdentity()
{
    $e = $this->getController()->getEvent();
    return $e->getParam('ZF\MvcAuth\Identity', new GuestIdentity());
}
```