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/types/types.go

@@ -552,7 +552,7 @@ func NewInviteResponse(event *types.HeaderedEvent) *InviteResponse {
 
 	// Then we'll see if we can create a partial of the invite event itself.
 	// This is needed for clients to work out *who* sent the invite.
-	inviteEvent := synctypes.ToClientEvent(event.Event, synctypes.FormatSync)
+	inviteEvent := synctypes.ToClientEvent(event.PDU, synctypes.FormatSync)
 	inviteEvent.Unsigned = nil
 	if ev, err := json.Marshal(inviteEvent); err == nil {
 		res.InviteState.Events = append(res.InviteState.Events, ev)

syncapi/types/types_test.go

@@ -56,7 +56,7 @@ func TestNewInviteResponse(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	res := NewInviteResponse(&types.HeaderedEvent{Event: ev})
+	res := NewInviteResponse(&types.HeaderedEvent{PDU: ev})
 	j, err := json.Marshal(res)
 	if err != nil {
 		t.Fatal(err)