Keep track of whether Destination supports HTTP/3 based on first server response

[cowwoc]

Jetty version(s)
12.x

Enhancement Description
Following up on the discussion at #11368 (comment) I would like to explore the idea of HttpClient issuing the first connection to a host using the HTTP/2, HTTP/1.1 preference order and if the server response indicates that the host supports HTTP/3 then subsequent requests to that host would use that protocol. Per the discussion, the server indicates HTTP/3 support using the Alt-Svc header or the ALTSVC frame.

@sbordet can you please clarify this point?

As I said, that implies an order relationship based on the protocol (HTTP/3.compareTo(HTTP/2) > 0) that may be true for browsers, but not for HttpClient.

Why is it that HTTP/3 is helpful for browsers but not necessarily HttpClient?

[sbordet]

Why is it that HTTP/3 is helpful for browsers but not necessarily HttpClient?

Because about half the web traffic is from mobile, but HttpClient is rarely used on mobile (although it can be).

Because the QUIC implementation we use (Quiche) is a native library that we need to call multiple times to process a request, so the performance is much worse than the other protocols.

Also, UDP may not be enabled in certain environments, so it is not desirable to even try HTTP/3.

The list continues.

Even for browsers may not be entirely true that HTTP/3 is faster/better than previous versions.