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.

clientapi/routing/createroom.go

@@ -466,7 +466,7 @@ func createRoom(
 		}
 
 		// Add the event to the list of auth events
-		builtEvents = append(builtEvents, &types.HeaderedEvent{Event: ev})
+		builtEvents = append(builtEvents, &types.HeaderedEvent{PDU: ev})
 		err = authEvents.AddEvent(ev)
 		if err != nil {
 			util.GetLogger(ctx).WithError(err).Error("authEvents.AddEvent failed")
@@ -536,7 +536,7 @@ func createRoom(
 			case spec.MRoomMember:
 				fallthrough
 			case spec.MRoomJoinRules:
-				ev := event.Event
+				ev := event.PDU
 				globalStrippedState = append(
 					globalStrippedState,
 					fclient.NewInviteV2StrippedState(ev),
@@ -558,7 +558,7 @@ func createRoom(
 			}
 			inviteStrippedState := append(
 				globalStrippedState,
-				fclient.NewInviteV2StrippedState(inviteEvent.Event),
+				fclient.NewInviteV2StrippedState(inviteEvent.PDU),
 			)
 			// Send the invite event to the roomserver.
 			event := inviteEvent