Add rich presence helper and Client.set_rich_presence

[mikasa]

Summary

This PR adds rich presence support for user accounts, mirroring how
discord.js-selfbot-v13 handles custom RPC:

• New discord.rich_presence.RichPresence builder class
  • Supports external assets via mp:external/...
  • Supports clickable images via large_url / small_url
  • Supports up to 2 buttons (labels in buttons, URLs in metadata.button_urls)
• New helper discord.rich_presence.get_external_assets(client, app_id, *urls)
  • Wraps POST /applications/{id}/external-assets
  • Returns keys usable as assets.large_image / assets.small_image
• New Client.set_rich_presence(rp, status="online", afk=False)
  • Sends OP 3 PRESENCE_UPDATE with the given rich presence activity

I'm down to add the docs if wanted.

General Info

• If code changes were made then they have been tested.
  • [] I have updated the documentation to reflect the changes.
• This PR fixes an issue (please put issue # in summary).
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[mikasa]

Probably worth mentioning, I didn't really look at the current activities setup and just decided to just make an entirely new helper which supports rich presence completely, including the clickable images.

Is there any particular way you would like me to document this?