MINOR: applet: Add callback function to deal with zero-copy forwarding

MINOR: applet: Add callback function to deal with zero-copy forwarding

This patch introduces the support for the callback function responsible to
produce data via the zero-copy forwarding mechanism. There is no
implementation for now. But <to_forward> field was added in the appctx
structure to let an applet inform how much data it want to forward. It is
not mandatory but it will be used during the zero-copy forwarding
negociation.