Creates a token that can be used to obtain a valid session for a particular signer of an existing package,
so they can sign the package. This single-use token is restricted to a single package and expires after 30 minutes
(this is configurable); however, it can be used multiple times. This token allows you to log in using a Get to the
%2Fauth?senderAuthenticationToken=VALUE%26target=http://environment/destination.
The destination must be a valid destination for a signer.