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.

roomserver/internal/query/query_test.go

@@ -84,7 +84,7 @@ func (db *getEventDB) EventsFromIDs(ctx context.Context, roomInfo *types.RoomInf
 	for _, evID := range eventIDs {
 		res = append(res, types.Event{
 			EventNID: 0,
-			Event:    db.eventMap[evID],
+			PDU:      db.eventMap[evID],
 		})
 	}