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/routing/relations.go

@@ -113,7 +113,7 @@ func Relations(
 	for _, event := range filteredEvents {
 		res.Chunk = append(
 			res.Chunk,
-			synctypes.ToClientEvent(event.Event, synctypes.FormatAll),
+			synctypes.ToClientEvent(event.PDU, synctypes.FormatAll),
 		)
 	}