Implement 100 Continue for HTTP/3

[adityamandaleeka]

Addresses #35974

Keeping this in draft while I confirm this is the expected behavior.
Update 9/9: client/server interoperability has been verified. This should be good.

[JamesNK]

@Tratcher In what situation does HttpClient send a 100 continue?

Is this something we can test end-to-end in the interop tests, or is it difficult to trigger 100 continue from HttpClient?

[Tratcher]

Sending Expect: 100-continue with HttpClient is easy.
httpRequestMessage.Headers.ExpectContinue = true

Triggering the 100 response on the server is easy, you just read the request body.

The real trick is observing that the 100 response happened, it's implicitly sent by the server and implicitly handled by HttpClient, the app code on either side never sees it.

Try this:
socketsHttpHandler.Expect100ContinueTimeout = TimeSpan.FromDays(1);
Then add a 10s timeout CT on the request body read calls.

This will prevent the client from sending the body until the 100 response is received. If the server doesn't send the 100 response then the client will not send the data and the server read will time out.

[adityamandaleeka]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/aspnetcore/actions/runs/1223025051