http3: add the qlog event schema to trace header (#5383)

integrationtests/self/self_test.go

@@ -137,6 +137,10 @@ func (t *multiplexedTrace) AddProducer() qlogwriter.Recorder {
 	return &multiplexedRecorder{Recorders: recorders}
 }
 
+func (t *multiplexedTrace) SupportsSchemas(schema string) bool {
+	return true
+}
+
 func getQuicConfig(conf *quic.Config) *quic.Config {
 	if conf == nil {
 		conf = &quic.Config{}