New request and response fields in multiple Playlists, Videos, and Channels APIs. New fields in a Surveys webhook event.

New request and response fields


## Changed
- New `thumbnails` request field in the following **Playlists** APIs:
	- `POST /v2/video_management/playlists`
	- `PATCH /v2/video_management/playlists/{playlistId}`
- New `thumbnails` response field in the following **Playlists** API:
	- `GET /v2/video_management/playlists`
- New `thumbnails`, `transcripts` request fields in the following **Videos** API:
	- `PATCH /v2/video_management/videos/{videoId}/metadata`
- New `thumbnails` response field in the following **Videos** API:
	- `GET /v2/video_management/videos/{videoId}/metadata`
- New `thumbnails` request field in the following **Channels** APIs:
	- `PATCH /v2/video_management/channels/{channelId}`
	- `POST /v2/video_management/channels`
- New `thumbnails` response field in the following **Channels** API:
	- `GET /v2/video_management/channels/{channelId}`
- New `quiz_name`,`video_name`  fields in the following **Surveys**  webhook events:
	- `video_management.video_quiz_completed`
