tools: ynl: add sample for ethtool

Configuring / reading ring sizes and counts is a fairly common
operation for ethtool netlink. Present a sample doing that with
YNL:

$ ./ethtool
Channels:
    enp1s0: combined 1
   eni1np1: combined 1
   eni2np1: combined 1
Rings:
    enp1s0: rx 256 tx 256
   eni1np1: rx 0 tx 0
   eni2np1: rx 0 tx 0

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>