net: ethtool: add support for structured PHY statistics

net: ethtool: add support for structured PHY statistics

Introduce a new way to report PHY statistics in a structured and
standardized format using the netlink API. This new method does not
replace the old driver-specific stats, which can still be accessed with
`ethtool -S <eth name>`. The structured stats are available with
`ethtool -S <eth name> --all-groups`.

This new method makes it easier to diagnose problems by organizing stats
in a consistent and documented way.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>