Add support for a WaitForPossibleDeletion

[remyleone]

Due to Apple licences, the M1-M servers need to be used for at least 24 hours. The API response gives in the fields of the server a field deletable_at that indicates when it is allowed to delete a server.
A WaitForPossibleDeletion would rely on the local clock to wait for the time at which it is permitted to delete a server. Then we could call a delete function normally.