netkit: Add add netkit {head,tail}room to rt_link.yaml

Add netkit {head,tail}room attribute support to the rt_link.yaml spec file.

Example:

  # ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_link.yaml \
   --do getlink --json '{"ifname": "nk0"}' --output-json | jq
  [...]
  "linkinfo": {
    "kind": "netkit",
    "data": {
      "primary": 0,
      "policy": "forward",
      "mode": "l3",
      "scrub": "default",
      "headroom": 0,
      "tailroom": 0,
      "peer-policy": "forward",
      "peer-scrub": "default"
    }
  },
  [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/bpf/20241220234658.490686-2-daniel@iogearbox.net