Add support for video_info field in MediaObj struct

[doziestar]

The current implementation of MediaObj in the media_obj package does not support the video_info field in the media object returned by Twitter's API. This field contains additional information about a video, such as its variants and duration, and is crucial for processing video media.

To address this, I propose adding a new struct called MediaWithVariant to represent media objects that contain a video_info field. This struct includes fields for the variants, duration_ms, and preview_image_url of the video, as well as a MediaPublicMetrics struct to represent public engagement metrics for the media content.

Additionally, I suggest modifying the existing MediaObj struct to only include fields that are common to all types of media, such as media_key, type, url, height, width, and MediaMetricsObj. This will ensure that the MediaObj struct remains lightweight and flexible, while still providing all the necessary information for non-video media.

I will submit a pull request shortly with these changes. Please let me know if you have any feedback or concerns.

Thank you.