Create `getInputFilter()` 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 create a `getInputFilter()` controller plugin, in order to mirror the API seen in REST resources.

```php
public function getInputFilter()
{
    $e = $this->getController()->getEvent();
    return $e->getParam('ZF\ContentValidation\InputFilter', null);
}
```