feature: ability to delegate the client signature verfication

[kumaakh]

While using lcc, I realized any complex form of client signature verification requires extension of some internal strategy classes.
I ended up implementing a simple callback process allowing the client-signature to be be verified by the application using other means: there are many usecase:
USB dongles
online verification etc.

pull request #178

[gcontini]

I'm back, finally have some time to restart this projects after many -- intense -- years elsewhere.
Yes you have a point, the library lacks of flexibility...

A mechanism that allow users to plug in their own classes is really needed, but first i need to restructure the library... and it's 8 years that i don't touch C++ it will be s. If you want to help, i'll be glad.

I saw your pull request but it's not how i want to do it. I do not like the dynamic registration mechanism. I was thinking to something "compile time" that allows you to switch default implementations with user provided ones.

Let me see if i can "savage" something from your PR :)

[mr-j0nes]

@gcontini good to see you back Gabriele!