RESPONDER: New interface for client registration
    
    This is just a beginning of new responder interface to data provider
    and it is just to make the client registration work. It needs further
    improvement.
    
    The idea is to take the existing interface and make it work better
    with further extensions of data provider. The current interface has
    several disadvantages such as it is originally build only for
    account requests and doesn't take different set of output parameters.
    It also doesn't work well with integration into tevent-made responders.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>