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.

federationapi/routing/invite.go

@@ -107,7 +107,7 @@ func InviteV1(
 func processInvite(
 	ctx context.Context,
 	isInviteV2 bool,
-	event *gomatrixserverlib.Event,
+	event gomatrixserverlib.PDU,
 	roomVer gomatrixserverlib.RoomVersion,
 	strippedState []fclient.InviteV2StrippedState,
 	roomID string,
@@ -198,7 +198,7 @@ func processInvite(
 	)
 
 	// Add the invite event to the roomserver.
-	inviteEvent := &types.HeaderedEvent{Event: &signedEvent}
+	inviteEvent := &types.HeaderedEvent{PDU: signedEvent}
 	request := &api.PerformInviteRequest{
 		Event:           inviteEvent,
 		InviteRoomState: strippedState,