Support for custom status

[Dorukyum]

Summary

discord/discord-api-docs#6345

What is the feature request for?

The core library

The Problem

API change

The Ideal Solution

Reflect the API

The Current Solution

No response

Additional Context

No response

[Lulalaby]

It's important that custom status support is added somewhat like that:

	public DiscordActivity(string name, ActivityType type)
	{
		if (type == ActivityType.Custom)
		{
			this.Id = "custom";
			this.Name = "Custom Status";
			this.State = name;
		}
		else
			this.Name = name;

		this.ActivityType = type;
	}