get_archived_public_threads and get_archived_private_threads use a u64 before instead of a timestamp

[BigBadE]

According to https://github.com/discord/discord-api-docs/blob/main/docs/topics/Threads.md#DOCS_RESOURCES_CHANNEL/list-public-archived-threads, getting archived threads, both public and private, are sorted by archived timestamp. The code instead takes an ID of some form, but the API expects an ISO timestamp. This can easily be fixed by simply switching before from an Option to an Option. Tested locally, and confirmed that this fix works.

It would also be nice if a version of the messages iterator was provided for public/private archived threads.