Add support for messages with type purchase_notification

[Soheab]

Summary

Relevant docs PR/Discussions:

• https://github.com/discord/discord-api-docs/pull/6927
• https://github.com/discord/discord-api-docs/discussions/6935#discussioncomment-9786723

Checklist

• [ ] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [ ] 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, ...)