Creates a token that can be used to obtain a valid session for the owner of an existing package, so they can edit
the package using the Designer view, and then distribute it for signing. The destination must be a valid
destination for a sender and is restricted to a particular package. It is single-use and expires after
30 minutes (this is configurable). This token allows you to log in using a Get to the
?senderDesignerToken=VALUE or ?senderTransactionEditToken=VALUE