Webhooks Create

[MagicTrixor]

Hey, thanks for this Calendly API Wrapper.

I noticed on Webhooks Create if the webhook already exists, it fails with a Fatal Error, because it returns a null and strict types is enabled and it expects a valid WebhookSubscriptionVO. One way to fix it would be by putting a ? before the return type, so null is allowed.
I didn't not verify any other ones, since I just starting using it.

I also will be using more endpoints in the near future, so if I catch something, I will let you know.