feat(Webhook): addition of Webhook#avatarURL function

[Fyko]

Please describe the changes this PR makes and why it should be merged:
This PR adds support for a Webhook#avatarURL function. Although not documented, a webhook's avatar follows the same format as a users.

I tested this with the following code in my eval command:

(async () => {
  const webhook = await msg.channel.fetchWebhooks().then(h => h.first());
  return this.client.rest.cdn.Avatar(webhook.id, webhook.avatar);
})()

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.