feat: Allow webhooks to fetch, edit and delete messages in threads

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
This will allow webhooks to fetch, edit and delete messages in threads.

Webhook#fetchMessage() unfortunately didn't already have an object for its second parameter. I would have liked to change it to what I commented here but to keep it not breaking, I instead added a new client option for webhooks that as a sort of opt-in feature flag that makes the library parse the second parameter as an object, thus allowing access to fetching a message in a thread.

Here's an example:

const { WebhookClient } = require("discord.js");

const webhook1 = new WebhookClient({
  url: "webhook_url1"
});

const webhook2 = new WebhookClient({
  url: "webhook_url2"
}, {
  allowWebhookThreadFetching: true // With this enabled, I can now fetch messages in threads!
});

webhook1.fetchMessage("message_id1", false).then(console.log); // Success!
// No error or change in code as we did not opt-in to fetching in threads. Parameter is unchanged.

webhook2.fetchMessage("message_id2", {
  cache: false,
  threadId: "thread_id"
}).then(console.log); // Success!

This should resolve #6592, additional. I am open to adjustments and feedback!

Status and versioning classification:

• 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
• This PR changes the library's interface (methods or parameters added)

[monbrey]

What's the point of the client option? Why would we ever NOT want to allow this?

EDIT: Okay, I missed the reason in your post, I see it now. Not a fan of that solution, its extremely monkey-patched.

[Jiralite]

I have gone with @ckohen's desired approach. Feel free to re-review!