http3: changing how http3 is built, doing early config validation.

[alyssawilk]

@mattklein123 is this a useful step in #12829 or no? If not I can just move the test elsewhere rather than deal with all the build complexity TODOs

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part of #14829

[mattklein123]

@alyssawilk this is roughly what I had in mind, but TBH I haven't looked through it in detail. Here is how I would personally approach it:

1. Search the code for any place source (not extensions) references QUIC/QUICHE extension code or well known names. I'm pretty sure there are some. Can we get rid of these easily?
2. Can we easily do cross config validation for all the pieces?

If we can't easily do (1) and (2) with everything being an extension I would do exactly what you are starting to do in this PR, and we can fully rip out all the extension loading stuff which I think will make things much simpler. Also see my other PR around the GSO batch writer. Let's land that other PR and then we can figure out what to do in aggregate?