Add support for video creation using image slideshows

Summary: Introduced slideshow_spec to AdVideo and added new file SlideshowSpecFields to hold sub-fields. slideshow can still be used against graph-video endpoint, so continue to do so in AdVideo creation but adjust to not require a source video file

Test Plan:

use FacebookAds\Object\AdVideo;
use FacebookAds\Object\Fields\AdVideoFields;

$video = new AdVideo(null, $ad_account_id);
$video->{AdVideoFields::SLIDESHOW_SPEC} = array (
  'images_urls' => array(
    $image_url_1,
    $image_url_2,
    $image_url_3,
  ),
  'duration_ms' => 2000,
  'transition_ms' => 200,
);

$video->create();

Author: Christine Lu

src/FacebookAds/Object/AdVideo.php

@@ -49,8 +49,11 @@ public static function getFieldsEnum() {
 
   public function create(array $params = array()) {
     $data = $this->exportData();
-    $source = $data[AdVideoFields::SOURCE];
-    unset($data[AdVideoFields::SOURCE]);
+    $source = null;
+    if (array_key_exists(AdVideoFields::SOURCE, $data)) {
+      $source = $data[AdVideoFields::SOURCE];
+      unset($data[AdVideoFields::SOURCE]);
+    }
     $params = array_merge($data, $params);
 
     $request = $this->getApi()->prepareRequest(
@@ -60,7 +63,9 @@ public function create(array $params = array()) {
     );
 
     $request->setLastLevelDomain('graph-video');
-    $request->getFileParams()->offsetSet(AdVideoFields::SOURCE, $source);
+    if ($source) {
+      $request->getFileParams()->offsetSet(AdVideoFields::SOURCE, $source);
+    }
     $response = $this->getApi()->executeRequest($request);
 
     $data = $response->getContent();

src/FacebookAds/Object/Fields/AdVideoFields.php

@@ -42,6 +42,7 @@ class AdVideoFields extends AbstractEnum {
   const NAME = 'name';
   const PICTURE = 'picture';
   const PUBLISHED = 'published';
+  const SLIDESHOW_SPEC = 'slideshow_spec';
   const SOURCE = 'source';
   const UPDATED_TIME = 'updated_time';
   const THUMBNAILS = 'thumbnails';