Add freely usable data pointer to "pappl_pr_driver_t" data structure

[tillkamppeter]

Is your feature request related to a problem? Please describe.
In my PostScript Printer Application I include PostScript Printer PPD files in the Snap, each PPD makes up a driver in the Printer Application. When a print queue is set up the appropriate PPD file is read and the pappl_pr_driver_t is appropriately populated.
The PPD file options not covered by IPP/PAPPL I want to add as vendor options. In addition to the IPP names for the options and choices and human-readable names (Issue #66) I also want to add some extra data needed for the print job execution.
For this I would simple need a freely usable data pointer in the pappl_pr_driver_t data structure which I could let point to my own data structure containing the extra data.

Describe the solution you'd like
A simple, freely usable data pointer in the pappl_pr_driver_t data structure.

[michaelrsweet]

This already exists and is called "extension".