Add new object op oo_id_attrs_get

Add new object op oo_id_attrs_get

The current oo_id_attrs nl_object op allows a fixed
id attribute list for an cache. But a cache with multiple families
may need to specify different id attributes for different families.

An example for this is the bridge fdb entries in the neigh cache:
neigh entries belonging to the AF_UNSPEC family use
(NEIGH_ATTR_IFINDEX | NEIGH_ATTR_DST | NEIGH_ATTR_FAMILY) as id attributes.
AF_BRIDGE fdb entries which also support the same msg type, will need to use
(NEIGH_ATTR_LLADDR | NEIGH_ATTR_FAMILY) as id attributes.
Today you cannot specify different set of attributes to two families belonging
to the same cache.

This patch adds a new object function oo_id_attrs_get to get the attributes.
An example implementation of oo_id_attrs_get for the neigh cache will
look like:
static uint32_t neigh_id_attrs_get(struct nl_object *obj)
{
        struct rtnl_neigh *neigh = (struct rtnl_neigh *)obj;

        if (neigh->n_family == AF_BRIDGE)
                return (NEIGH_ATTR_LLADDR | NEIGH_ATTR_FAMILY);
        else
                return (NEIGH_ATTR_IFINDEX | NEIGH_ATTR_DST | NEIGH_ATTR_FAMILY);
}

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: Nolan Leake <nolan@cumulusnetworks.com>
Reviewed-by: Shrijeet Mukherjee <shm@cumulusnetworks.com>
Reviewed-by: Wilson Kok <wkok@cumulusnetworks.com>