introduce Transport.ConnContext, use client's context on the connection (#4507)

* introduce Transport.ConnContext, use client's context on the connection

* panic if ConnContext returns nil

transport.go

@@ -89,6 +89,16 @@ type Transport struct {
 	// implementation of this callback (negating its return value).
 	VerifySourceAddress func(net.Addr) bool
 
+	// ConnContext is called when the server accepts a new connection.
+	// The context returned from the callback is used to derive every other context used during the
+	// lifetime of the connection:
+	// * the context passed to crypto/tls (and used on the tls.ClientHelloInfo)
+	// * the context used in Config.Tracer
+	// * the context returned from Connection.Context
+	// * the context returned from SendStream.Context
+	// It is not used for dialed connections.
+	ConnContext func() context.Context
+
 	// A Tracer traces events that don't belong to a single QUIC connection.
 	// Tracer.Close is called when the transport is closed.
 	Tracer *logging.Tracer
@@ -168,6 +178,7 @@ func (t *Transport) createServer(tlsConf *tls.Config, conf *Config, allow0RTT bo
 		t.conn,
 		t.handlerMap,
 		t.connIDGenerator,
+		t.ConnContext,
 		tlsConf,
 		conf,
 		t.Tracer,