rtnetlink/neigh: add new_bridge helper to add AF_BRIDGE neighs

[Tuetuopay]

This allows to manipulate the FDB in addition to the ARP table.

While the same could be accomplished with .message_mut(), this is less cumbersome:

Before:

// 0.0.0.0 as a dummy IP, we don't need it for generic FDB entries
let mut req = handle.neighbours()
    .add(index, [0, 0, 0, 0].into())
    .state(NUD_PERMANENT)
    .link_local_address(&[0x8c, 0x8c, 0xde, 0xad, 0xbe, 0xef]);
let mut msg = req.message_mut();
msg.header.family = AF_BRIDGE as u8;
req.execute().await?;

After:

handle.neighbours().add_bridge(index, &[0x8c, 0x8c, 0xde, 0xad, 0xbe, 0xef]).execute().await?;

[little-dude]

Thanks, looks neat. I'll wait for tomorrow before merging in case someone else wants to take a look.