Make an "afk" support for the PRESENCE_UPDATE event

[kingmigdor]

As far as I can see for now, clients can send an "afk" with its status to update. Why can't bots receive that?

General use-cases for now:

• bots can manage afk users per-voicechannel again instead of having one setting in the server settings. There was a bot before that was able to manage it before the setting was added iirc. it would also be possible to make channels not move members to afk if the user goes afk. it would also be possible to move a user to audience in stage channels if they are afk and on the podium.
• bots can lock channels for people who are afk and require a password to rejoin. there are several people who'd like to keep their conversation as most private as they can, normally you can use private text channels but this can go further
• there are bots with an /afk command to make the bot reply to a ping that happens to the user. this could be persistent and a value that is really sending if the user is really afk
• it could also be possible to count "active" accounts instead of "online" accounts, which checks !member.afk. there are still some servers dealing with inactivity or not knowing when users are actually active - this can be quite different to when users are online.
• general userinfo: yes, it could also be possible to implement that information in a simple userinfo command showing up whether the user is afk or not.

Alternatives considered:

• it would be possible to check PRESENCE_UPDATE if a user moves from an online status to an idle status. This is inconsistent and could be manually set by users, because users may have set idle because it looks nice or something.
• it can be possible to check if a user is afk if the user is in the afk channel. I know some bots doing that but this limits people to be in a voice channel. I also think that to check this bots would need guild members intent which should definetely be moved to the presence intent in my opinion.