Add support for reviewers in Pull Requests for GitHub-compatible webhook

[gnirb]

Hi!

First of all, thanks for the awesome GitHub-compatible webhook provided in https://discord.com/developers/docs/resources/webhook#execute-githubcompatible-webhook, it saves us from having to specify a custom GitHub actions job for each repository to get notifications on Discord.

However, I do miss one relevant detail in our notifications for Pull Requests - It doesn't display who the reviewers are, so our whole team keeps clicking the PR notifications even if only one member is assigned.

The required data is already provided by GitHub in their webhook requests, so the only change would be to include it (if available) in the notification produced by Discord.

Sample GitHub webhook pull_request event data:

{
  "action": "opened",
  ...
  "pull_request": {
    "requested_reviewers": [
      {
        "login": "gnirb", <-- I would like this displayed in notification
        ...
      },
   }
}

It documented by GitHub here: https://docs.github.com/developers/webhooks-and-events/webhooks/webhook-events-and-payloads#pull_request