feat(message): add the new ex query param to Attachment with some util property

[Snipy7374]

Summary

Mostly waiting for docs

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, ...)

[shiftinv]

Thanks for the PR! It seems unusual to retain Attachment objects for several days, handling the possible expiry feels like something that should be done in the application rather than the library.
The expiry on new attachments is currently ~14 days, and attachments returned by the API (e.g. from fetching a message) get refreshed automatically and should always be valid in that moment; therefore this shouldn't concern most people.