Support the WEBHOOKS_UPDATED event

[CottageDwellingCat]

The WEBHOOKS_UPDATE event is ignored by socket clients, and a comment claims it is user only, however it is documented and other libraries appear to support it. The event is useful in webhook caching solutions that need to know if a webhook was moved.
The ignored event is found at:

Discord.Net/src/Discord.Net.WebSocket/DiscordSocketClient.cs

Lines 2861 to 2864 in a890de9

	case "WEBHOOKS_UPDATE":
	await _gatewayLogger.DebugAsync("Ignored Dispatch (WEBHOOKS_UPDATE)").ConfigureAwait(false);
	break;
	#endregion

Example using the event:

client.WebhooksUpdated += (guild, channel) =>
{
    Console.ForegroundColor = ConsoleColor.Magenta;
    Console.WriteLine($"A webhook was updated in or moved to {guild}#{channel}");
    Console.ResetColor();
    return Task.CompletedTask;
};