new classes: PartialWebhook and possible PartialSyncWebhook

[onerandomusername]

Summary

new classes: PartialWebhook and possible PartialSyncWebhook

What is the feature request for?

The core library

The Problem

Webhooks currently aren't too clear that they are partial or full webhooks.

The Ideal Solution

PartialWebhook and PartialSyncWebhook classes.

PartialWebhook should subclass Webhook.

All of the attributes only accessible with a full Webhook would be typed as None.
PartialWebhook.fetch() would return a Webhook instance, and Webhook.from_url() would be changed to return a PartialWebhook

These changes would also be made to SyncWebhook with PartialSyncWebhook.

The Current Solution

check if an attribute is None to know if this Webhook is full or partial.

Additional Context

^{this may be able to be implemented differently, @shiftinv}