Add webhook event builder

[odolbeau]

Would you consider a PR containing a WebhookEventBuilder class which will transform a JSON string to an Event?

List of webhooks events classes:

• Archive
• ConnectionCreated
• ConnectionDestroyed
• StreamCreated
• StreamDestroyed

The stripe SDK propose something similar (but with a generic event class).

[SecondeJK]

We have something similar to this in the Vonage PHP SDK Core (Comms APIs)
If the PR followed this pattern (which hydrates incoming webhooks via. PHP globals) then I'd be up for merging it