Add a way to custom BootstrapHandler behavior at server side

[sbernard31]

This PR allows to customize bootstrap server behavior like this:

builder.setBootstrapHandlerFactory(new BootstrapHandlerFactory() {
    
    @Override
    public BootstrapHandler create(BootstrapStore store, LwM2mBootstrapRequestSender sender,
            BootstrapSessionManager sessionManager) {
        return new MyBootstrapHandler(store, sender, sessionManager);
    }
})