http3: set Early-Data header for 0-RTT requests, automatically retry on 425

[marten-seemann]

See https://datatracker.ietf.org/doc/html/rfc8470#section-5.1.

[marten-seemann]

I don't understand this section of the RFC:

The Early-Data header field is not intended for use by user agents
(that is, the original initiator of a request). Sending a request in
early data implies that the client understands this specification and
is willing to retry a request in response to a 425 (Too Early) status
code. A user agent that sends a request in early data does not need
to include the Early-Data header field.

Is quic-go a user agent?

[tanghaowillow]

@marten-seemann From my understanding, the Early-Data header is for intermediary(e.g. an API gateway in general scene) when forwarding a request. So the server behind the intermediary can know that the original request is sent in 0-RTT data even when the request is forwarded in protocol like HTTP/1.1.
So I think this is not a feature that should be imlemented in quic-go layer. People should implemnet it at the HTTP layer if they are building an intermediary service over quic-go though.
Neither the retry on 425 feature should not be added in quic-go I believe, as the a http.RoundTripper (http3.Transport in quic-go) should only be responsible executing single HTTP transaction, which means logics like redirection and retry should be implemented at higher layer.