The GetNotificationChannelVerificationCode request.
The desired expiration time. If specified, the API will
guarantee that the returned code will not be valid after the
specified timestamp; however, the API cannot guarantee that
the returned code will be valid for at least as long as the
requested time (the API puts an upper bound on the amount of
time for which a code may be valid). If omitted, a default
expiration will be used, which may be less than the max
permissible expiration (so specifying an expiration may extend
the code's lifetime over omitting an expiration, even though
the API does impose an upper limit on the maximum expiration
that is permitted).
