Changes to decouple handling functionality from the controller

[shmulyeng]

In using this library, I came across the following two requirements.

1 - Handle all events generically (I'm logging all events sent to the webhook)
2 - Decouple the handling functionality from the controller so that I can call it independently.

I made the coding changes to support this. I also made a change that each handler function can return an Ok or a BadRequest.

Is this something that would be of interest to the public? If yes, I can submit a PR.

[staviloglu]

@shmulyeng let's wait for the 👍 from other users of the sdk. When it reaches a significant amount, we can add that feature.

[shmulyeng]

I'm also making the changes to support Folder webhooks. I guess we'll see if there's any interest in that.