[Feat] Implement callback registration for route handling

[PrashanthaTP]

User of the api should be able to register callbacks which will be invoked by the server,
whenever the client requests for a rgistered path using registered method

• Callback Type -> typedef ? using?
• Create Request obj out of raw request data
• Pass Response to callback
• Copy Response obj data to raw response to send it to client
• Use register api in main
• Fix [Bug] Callback registered for route handling not working? #11