Hi, I want to add the missing `Topics` field for the [EditChange](https://github.com/google/go-github/blob/master/github/event_types.go#L390) `struct`. In my case, it will allow me to retrieve the previous topics when I receive the [repository edited event](https://docs.github.com/en/webhooks/webhook-events-and-payloads?actionType=edited#repository). I already have the branch ready for review. Thanks