[eporner] Add support for AV1 codec

[michal-repo]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

Some videos are available with AV1 codec, that information is not returned in JSON from xhr/video/{video_id} but can be extracted from webpage. URL to download is identical as h264 file but with addition of -av1 before file extension (.mp4).

Fixes #

Add AV1 codec

Before submitting a pull request make sure you have:

• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check all of the following options that apply:

• I am the original author of this code and I am willing to release it under Unlicense
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What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)