[TikTok] Add support for downloading TikTok video cover via config key

[Nimble8213]

Feature Request

Background:
Currently, TikTok video extraction in gallery-dl does not provide an option to download the video cover image directly. When using gallery-dl -J, the TikTok extractor returns the cover as video.originCover in the JSON response.

Requested Feature:

• Introduce a configuration key under the TikTok extractor section to enable downloading the video cover (originCover).
• When this key is enabled, the TikTok extractor should download and save the cover image alongside the video file.

Example:

{
  "extractor": {
    "tiktok": {
      "cover": true
    }
  }
}

Notes:

• The key should be clearly documented in the configuration options for the TikTok extractor.
• Implementation should fetch and save the originCover when the new config key is enabled.
• Should maintain backward compatibility for users not enabling this option.

Motivation:
This feature will allow users to easily download TikTok video covers for archiving, preview, or cataloging purposes without post-processing JSON results manually.

[FecklessFeck]

There are also "cover" and "dynamicCover". They might point to high quality "preview" thumbnails.
"yt-dlp" can actually download high quality covers with "--write-all-thumbnails" command.