Support reason in UsersService.Unsuspend #4613

UsersService.Unsuspend cannot send a reason.

The GHES endpoint Unsuspend a user (DELETE /users/{username}/suspended) accepts an optional JSON body with a reason field, which is recorded in the audit log. UsersService.Unsuspend always sends nil, so callers can't provide one and GitHub logs the default "Unsuspended via API by SITE_ADMINISTRATOR".

UsersService.Suspend already supports this through UserSuspendOptions.Reason (added in #1112, being renamed to SuspendUserRequest in #4612), but Unsuspend was left out.

I'd like to add an UnsuspendUserRequest with a Reason field and pass it by value to Unsuspend, matching Suspend. I'm planning to work on this.