Support enumerating the known event types

[evankanderson]

It looks like go-github has an internal list of all the GitHub event types . I'd like to be able to iterate over the list in my own code where I route events based on event type. I'm thinking of exposing the following methods:

// WebhookTypes returns a list of all the known GitHub event type strings supported by go-github.
func WebhookTypes() []string {
  // ...
}

// EventForType returns an empty struct matching the specified GitHub event type.
func EventForType(messageType string) any {

}

The second method would extract a lot of Event.ParsePayload https://github.com/google/go-github/blob/master/github/event.go#L30, so it wouldn't be much additional code.

[gmlewis]

I have no problem with that as long as you can refactor to reuse existing code as much as possible.
I say this because any time a new PR comes in to support a new webhook event type, it is a bit of a pain to make sure you tweak all the various places that need tweaking, and it would be nice to not add more pain to this process.