Add Video and Image diemensions field in respective class

Both classes ImageConversationCard and VideoConversationCard
have field which have data related to dimensions/size of Video/Image
but these fields despite present in API are being dropped/ignored in
class properties/attributes, thus consumer of this library can't
assess data like 'image_display_height' on ImageConversationCard till
now. Current fix enable the same.

lib/twitter-ads/creative/image_conversation_card.rb

@@ -35,6 +35,8 @@ class ImageConversationCard
       property :third_cta
       property :third_cta_tweet
       property :title
+      property :image_display_height
+      property :image_display_width
 
       RESOURCE_COLLECTION = "/#{TwitterAds::API_VERSION}/" \
                             'accounts/%{account_id}/cards/image_conversation' # @api private

lib/twitter-ads/creative/video_conversation_card.rb

@@ -37,6 +37,8 @@ class VideoConversationCard
       property :third_cta_tweet
       property :title
       property :media_key
+      property :video_width
+      property :video_height
 
       RESOURCE_COLLECTION = "/#{TwitterAds::API_VERSION}/" \
                             'accounts/%{account_id}/cards/video_conversation' # @api private