[BREAKING] Make `WebBrowser` and `ArchiWebHandler` support cancellation

[JustArchi]

Checklist

• [X] I read and understood ASF's Contributing guidelines
• [X] I also read Setting-up and FAQ, I don't need help, this is an enhancement idea
• [X] My idea doesn't duplicate existing ASF functionality described on the wiki
• [X] I believe that my idea falls into ASF's scope and should be offered as part of ASF built-in functionality
• [X] My idea doesn't violate the Steam Subscriber Agreement
• [X] My idea doesn't violate the Steam Online Conduct
• [X] This is not ASF-ui suggestion

Enhancement purpose

Underlying HttpClient supports cancellation, allow callers to use it if they need to.

Solution

CancellationToken cancellationToken = default + passing it around in all our code.

Why currently available solutions are not sufficient?

There is no reason why callers shouldn't be able to use this mechanism if they want to.

Can you help us with this enhancement idea?

Yes, I can code the solution myself and send a pull request

Additional info

Breaking change for all inheritors, don't ship before .NET 8 when we break more things.