[networking/rh] Add strict extension checkinge

[coletdjnz]

This adds strict checks for extensions in RequestHandlers.

Previously the logic meant any extension received was optional to support. However, this causes issues if an extension is required to be supported for the request.

This PR changes it so RequestHandlers have to specify what extensions they support. If they don't support an extension then the request is marked as unsupported by the handler. This is done through removing extensions from a copy of the extensions dict when checking.

Since the architecture is very new, compat is not a concern yet.

I won't do it as part of this PR, but if we need the support for optional extensions there is lots of ways we can go about adding it if need be, such as :

• extensions starting with underscore
• __optional__ extension containing list of optional extensions to ignore

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Copilot Summary

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Summary

🌐🧪🔧

This pull request adds support for request extensions to the RequestHandler class and its subclasses, which are used to perform network requests in yt-dlp. It also refactors the extension validation tests in test_networking.py to use a parametrized method.

Oh we are the coders of the sea
We write our RequestHandlers with glee
We check our extensions on the count of three
And raise an UnsupportedRequest if we disagree

Walkthrough

• Add and document _SUPPORTED_EXTENSIONS attribute to RequestHandler and its subclasses to validate request extensions (link, link, link, link)
• Implement core extension check in _check_timeout_extension method of RequestHandler to raise UnsupportedRequest exception for unsupported extensions (link)
• Refactor and parametrize extension tests in test_networking.py using EXTENSION_TESTS attribute and test_extension method (link, link)