pam-extensions: Add support for generic GDM CustomJSON binary protocols

GDM allows passing data through PAM binary conversations, however it only supports a private protocol for choice list.

Due to the GDM architecture, creating a new private protocol is quite verbose and it implies changing multiple layers, at the same time it is impossible for modules and GNOME Shell GDM/login extensions to easily define specific protocols to perform custom actions, and they currently need to rely on DBus or other ways to communicate without an easy way to setup a secure communication channel.

At the same time, extending a binary protocol it is hard, once one has been defined.

To make this easier to manage both by modules and by gdm clients, add simple JSONbased private protocols that are easy to extend and that moves the protocol definition at higher level than gdm, leaving to the modules and to gdm clients (the shell) the control over them.