allow custom callback for checking recv messages

[ptitdoc]

This allow custom use cases for bromelia such as diameter apps that answer to multiple names/aliases, specific logging of recv messages, or implementing diameter messages proxying.

The simplest use case it is to actually disable node recv message (which I use for a diameter proxy as I don't care about the destination node, I just forward messages to the next peer):

def diameter_recv_callback(association, message):
    
    print("RECV callback: ", association)

BaseMessageProcessor.set_recv_callback(diameter_recv_callback)

[ptitdoc]

Hello,

Are you interrested by this commit ? The goal for me is to write a diameter proxy. I have working code but I don't really like it, I can publish it along anyway if you want it for reference.