feat: soundboard

[shiftinv]

Summary

discord/discord-api-docs#6260

Depends on #993, but can be separated easily - the two PRs can really be merged in any order.

• As usual, naming is still tbd in a few places, but overall this PR is fully functional
• The current implementation just uses application/octet-stream instead of trying to determine the correct mime type, which seems to be working fine; may want to improve that, though
  • This also means that sound.to_file() does not contain a file extension, unless it's a default sound (which already has the file extension in its name)
• It's not entirely clear whether GUILD_SOUNDBOARD_SOUNDS_UPDATE contains the full list of sounds, or just a subset of modified ones

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• 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, ...)