Webhooks

[avignd]

Webhooks are a neat way to get updates by push to your web application.

The wikipedia article is pretty dry, so I will quote a blog post by Zapier:

Webhooks are one way that apps can send automated messages or information to other apps. It's how PayPal tells your accounting app when your clients pay you, how Twilio routes phone calls to your number, and how WooCommerce can notify you about new orders in Slack.

As the next release (0.6) is going to support metadata versioning and an eventing framework, now is a good time to support
webhooks for event changes on entities.

The general plan is to have an API or configuration where the admin can register his URL to recieve the webhooks, and if a URL is configured then have an Event Handler (which is invoked when ever there is a change to an entity) that publishes those events with a POST request.

Once this is done we should consider having a retry mechanism for requests that failed and support basic authentication.

Thoughts?

[sureshms]

@avignd here is the issue to track it - #1145