Update WebTransport handshake

[rlyshw]

/wt endpoint wasn't working. Needed to modify header negotiation to work again.

Changes mostly come from here:
https://github.com/GoogleChrome/samples/blob/gh-pages/webtransport/webtransport_server.py

[codecov]

Codecov Report

Merging #251 (3fb34f6) into main (9e360a5) will not change coverage.
The diff coverage is n/a.

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  Coverage   100.00%   100.00%           
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  Files           23        23           
  Lines         4572      4572           
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  Hits          4572      4572           

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[jlaine]

This doesn't look great, if H3Connection needs to be updated for recent protocol changes let's just change it there, not in the examples. Otherwise anybody wanting to use H3Connection will need to apply these hacks. Also I suspect the client suffers from the same problem.

Also I don't understand the need for H3_DATAGRAM_05, H3_DATAGRAM was already updated in aa8a038

So in essence I think the only thing this PR actually changes is enabling WebSockets?

[jlaine]

AFAICT most of the changes in this PR are not required. The only addition required for the WebTransport handshake to succeed is adding the sec-webtransport-http3-draft header in the server's response. This is done in 24eca50.

[jlaine]

FYI, with the changes in version 0.9.19 (just released) the Google Chrome example can be simplified, I've submitted a PR to that effect in GoogleChrome/samples#764

@yutakahirano

[yutakahirano]

Thank you!

[jlaine]

You're welcome @yutakahirano, it's in my interest to keep your examples as lean as possible, it reduces the noise on the issue tracker :)