Ability to set `NLM_F_APPEND` flag when adding `bridge fdb` neighbors

[mepholic]

I need the ability to set NLM_F_APPEND to append neighbors to the bridge fdb table. Appending to the bridge FDB table is required to add multiple L3 flood destinations to a vxlan interface. I'm currently using the add_bridge API. Upon inspecting the code in that file, I see that you can call replace, which causes the execute method to change NLM_F_EXCL to NLM_F_REPLACE in the final netlink request message.

I was about to open a PR to add append functionality when I saw #52, which is doing the same work, but for ip route instead of ip neigh/bridge fdb. I see this issue is waiting for #57 to be merged as well, and seeing as there is recent activity on this issue, I'm going to wait until this is merged before working on the feature that I need.

I'm also wondering at this point if NLM_F_APPEND is even valid for ip neigh entries, and if it makes sense for it to be combined with bridge fdb. In the networking world, the ARP/NDP tables (ip neigh) are related, but distinct from a switches forwarding table (bridge fdb)

[mepholic]

So I just actually looked at the implementation in #57 and see that it is only adding a RouteMessageBuilder, and there is no equivalent Message Builder implementation for neighbors. Should there be?

I guess I won't be waiting for #57 to be merged after all. For now, I'm just going to do the simplest possible fix for the neighbour implementation, which is adding an append method to add.rs. If anyone has any thoughts, please feel free to share 😄