netlink: specs: define ethtool header flags

When interfacing with the ethtool commands it's handy to
be able to use the names of the flags. Example:

    ethnl.pause_get({"header": {"dev-index": cfg.ifindex,
                                "flags": {'stats'}}})

Note that not all commands accept all the flags,
but the meaning of the bits does not change command
to command.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240403023426.1762996-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>