lib: add an exception in the northbound for operational data callbacks

lib: add an exception in the northbound for operational data callbacks

During initialization, the northbound detects if any required
callback is missing (fatal error) or if any unneeded callback is
present (warning).

There are three callbacks, however, that should require special
handling: get_next(), get_keys() and lookup_entry().

These callbacks are normally unneeded for configuration lists. But,
if a configuration list is augmented with new state nodes by another
module, then the three callbacks mentioned above become required. In
this case, never log a warning when these callbacks are implemented
when they are not needed, since this depends on context (e.g. some
daemons might augment "frr-interface" while others don't).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>