prometheus-node-exporter-lua: Add ethtool exporter

prometheus-node-exporter-lua: Add ethtool exporter

Based on ethtool-lua library, add interface statistics to the
prometheus-node-exporter.

As closely as possible, the behaviour of the implementation at
https://github.com/prometheus/node_exporter/blob/400c3979931613db930ea035f39ce7b377cdbb5b/collector/ethtool_linux.go#L208
is replicated.

Signed-off-by: Kevin Jilissen <redacted>
[squash commits, bump version, fix permission, use untyped, rename to ethtool]
Signed-off-by: Etienne Champetier <redacted>