Draft: Add a network access callback to check before making network connections to validate certificates

I added a pointer to a function in the PDFDoc class to be used as network access callback. To be able to verify certificates this pointer needs to be set to a function that takes no arguments and returns true or false depending if we want the library to make network connections or not. pdfsig utility was also modified to use the new callback by default except if -nocert argument is passed in the command line.

This is a work in progress, I don't know if modifying the PDFDoc class is the best place to add such a callback function but this is the most generic object class that is also accessible from FormFieldSignature class.