Support unknown webhook events

[danbillson]

Related to issue: #80

There are some webhook events that are no longer supported but there is still a use for them, some of the other SDKs have the concept of a generic event which is not explicitly handled or typed, this PR also introduces a GenericEvent which will not have any typing so it will be on the implementation to ignore or cast the types.

The recommended usage for this will be to handle the webhook as you would anything else but intervene with any ts suggestions i.e.

    switch (event?.eventType) {
      case EventName.SubscriptionCreated:
           // handle subscribtion created event

      // @ts-ignore
      case "invoice.paid":
          // handle invoice paid event as a generic event

To follow inline with the rest of the webhook events, the keys of the event object have been completely transformed to use camel case