Pass context to event publishing.

To allow tracing propagation via events, we need to pass the context
to the `Publish` function of reva events. This adds the context
everywhere where events are published. If there was no context to pass,
we started a new one with `context.Background()`.

services/policies/pkg/service/event/service.go

@@ -72,7 +72,7 @@ func (s Service) Run() error {
 				}
 			}
 
-			if err := events.Publish(s.stream, events.PostprocessingStepFinished{
+			if err := events.Publish(context.Background(), s.stream, events.PostprocessingStepFinished{
 				Outcome:       outcome,
 				UploadID:      ev.UploadID,
 				ExecutingUser: ev.ExecutingUser,