[ProVerif] Add more convenient way to include handwritten models

[jschneider-bensch]

While the ProVerif backend at the moment allows marking items for manual modelling, it does not offer a convenient way to integrate a set of handwritten models after extraction.

To address this, the pv_handwritten attribute should be extended to (optionally) include perhaps a filename where a handwritten model for the given item is specified. This can then be inserted in the extraction in a way that allows automatic replacement of the filename indicator with the handwritten model.