stream: Adding new NextEventRaw method

This methode allow go-concourse users to get raw sse.Event from the
eventstream.

This offer an handy way to fully get an event (payload and event_id), Instead of
a decoded atc.Event.

This function also provide a workaround for concourse#8585

go-concourse/concourse/eventstream/stream.go

@@ -15,6 +15,7 @@ import (
 //counterfeiter:generate . EventStream
 type EventStream interface {
 	NextEvent() (atc.Event, error)
+	NextEventRaw() (sse.Event, error)
 	Close() error
 }
 
@@ -50,6 +51,11 @@ func (s *SSEEventStream) NextEvent() (atc.Event, error) {
 	}
 }
 
+func (s *SSEEventStream) NextEventRaw() (sse.Event, error) {
+	se, err := s.sseReader.Next()
+	return se, err
+}
+
 func (s *SSEEventStream) Close() error {
 	return s.sseReader.Close()
 }