Use PDU not *Event in HeaderedEvent (#3073)

Requires https://github.com/matrix-org/gomatrixserverlib/pull/376

This has numerous upsides:
 - Less type casting to `*Event` is required.
- Making Dendrite work with `PDU` interfaces means we can swap out Event
impls more easily.
 - Tests which represent weird event shapes are easier to write.

Part of a series of refactors on GMSL.

syncapi/consumers/roomserver.go

@@ -526,7 +526,7 @@ func (s *OutputRoomEventConsumer) updateStateEvent(event *rstypes.HeaderedEvent)
 		PrevSender:    prevEvent.Sender(),
 	}
 
-	event.Event, err = event.SetUnsigned(prev)
+	event.PDU, err = event.SetUnsigned(prev)
 	succeeded = true
 	return event, err
 }