feat: new message type PURCHASE_NOTIFICATION

[EmmmaTech]

Summary

Implements GH-1225 & relevant types from the OpenAPI spec: Message.purchase_notification & PurchaseNotification.

Since these data types are not officially documented yet, I will put this PR on draft.
(Note: I am aware that attributes specified in the OpenAPI spec are technically public for anyone to implement, however I do not feel comfortable having the PR merged now without any explicit docs outlining this feature.)

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, ...)