Add NeighborMessageBuilder

[dvaembit]

I'm trying to write a test case for some code that uses rtnetlink. I need to construct a NeighborMessage from netlink-packet-route. Since that is marked as #[non_exhaustive] I cannot construct one directly. I am constructing a default message and mutate it like so:

let mut msg = NeighborMessage::default();
msg.headers = ...;
msg.attributes = ...;

However when I look through the documentation I see a LinkMessageBuilder for LinkMessage and a RouteMessageBuilder for RouteMessage. It makes me expect to see a NeighborMessageBuilder. Should there be?

[cathay4t]

It should, but my project does not use this NeighborMessage yet, so I have no time or motivation for this yet.

PR is always welcome.