[YouTube] Realign n-sig processing with yt-dlp

[dirkf]

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• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

This PR revises the processing of the n-sig YT parameter in the YoutubeIE extractor to realign the code with that in yt-dlp.

Some additional methods from the yt-dlp InfoExtractor class have already been back-ported in yt-dlp; this PR adds further such methods needed by the unified code.

Finally, some obvious test failures are fixed, including a new extraction tactic for like_count from likeButtonViewModel in the YT page, as recently implemented in yt-dlp (thanks @Ganesh910, yt-dlp/yt-dlp#8763).