Access to neighbor cache from interface API

[strtok]

In previous versions it was possible to pre-fill the neighbor cache. It looks like it's part of the inner Interface context() now, but no longer exposed to the public API? Am I missing an API call?

[thvdveld]

The access to the neighbour cache was removed in #722, where we changed from using managed to heapless. It was possible to prefill a neighbour cache because of the usage of managed. I guess we could add something like an update_neighbor_cache function, like how we do it for IP addresses. To be honest, I don't see why you would pre-fill a neighbour cache as this might come back as a footgun.