Webhook event support

[gingershaped]

Summary

The library should support Discord's outgoing HTTP webhook system.

What is the feature request for?

The core library

The Problem

Discord recently added support for events sent through a HTTP webhook, a system which is not presently supported by this library; this webhook is currently the only way to receive events for an app being authorized, and more events may be added that are exclusive to it in the future.

The Ideal Solution

This library should support running a HTTP server that can listen to events from this webhook, reply to pings, and dispatch events received through an idiomatic interface. This could either be added as its own independent class or as extra functionality in Client.

The Current Solution

Apps that want to listen to these events must write their own handlers and handle running an HTTP server themselves.

[MCausc78]

HTTP interactions already were rejected before, so webhook events will be rejected too. discord.py heavily depends on cache and gateway so this is unlikely to happen.