Add friend request callbacks

the goal is to provide a way to register callback for 'friend request' event. Before that one had to redo all the iq check that this plugin was already doing (and was simply silently ignoring them). Though i'm aware it's not strictly speaking directly related to roster manipulation. as it's a "pre-step" , it's related to roster, doing the check integrate quite well with current code, and any resulting action from the user would result in use of functions provided by this plugin, I think it perfectly make sense to provide it.