lib: introduce new 'pre_validate' northbound callback

lib: introduce new 'pre_validate' northbound callback

This callback can be used to validate subsections of the
configuration being committed before validating the configuration
changes themselves. It's useful to perform more complex validations
that depend on the relationship between multiple nodes.

Only YANG-level validation (performed by libyang) and the
NB_EV_VALIDATE validation (that can be used to validate individual
configuration changes) proved to be insufficient in some cases.

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