Is your feature request related to a problem? Please describe

It is impossible to determine, in advance, whether a target server supports HTTP/3. It is also impossible to upgrade an existing HTTP/1.1 or HTTP/2 connection to an HTTP/3 connection, since HTTP/1.1 and HTTP/2 are built on top of TCP streams while HTTP/3's QUIC is built on top of UDP datagrams.

Describe the solution you'd like

By default, if enabled, HTTP/3 transport is available on the same port as HTTP/1.1 and HTTP/2 transports. The Alt-Svc header (see please [1]) is the standard way to advertise alternative services (HTTP/2 or HTTP/3).

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Alt-Svc

Related component

Other

Describe alternatives you've considered

None

Additional context

Part of #4451