JSON schema prototype

[sanyarnd]

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What is the purpose of your pull request?

• Improvement
• New feature

Currently youtube-dl JSON output has no schema, information about JSON is scattered across doc-strings and markdown-docs and it's was cumbersome to write bindings for it.

Another point is that the lack of schema does not guarantee any comparability, though I doubt there would be significant changes, but it's nice to have anyway.

There was an attempt to submit schema in the past (#16353), but that schema doesn't cover whole json output and not really structured. I think it's better to keep separate PRs, yet I don't mind moving discussion to that topic.
Also I don't mind collaboration with the dev team and cut some unnecessary parts if required or add some missing parts or any work on python side.

You can run tests for validation online here, but for some reason site does not understand $schema keyword -- simply remove it. I don't know why they don't support it, because it's the strongly recommended to have this field.