prometheus-node-exporter-ucode: add nat_traffic collector

prometheus-node-exporter-ucode: add nat_traffic collector

Add a nat_traffic collector for prometheus-node-exporter-ucode,
ported from the equivalent Lua collector in prometheus-node-exporter-lua.

Reads /proc/net/nf_conntrack and exports per-(src, dst) pair byte
totals (both directions summed) as node_nat_traffic gauge metrics.

Also includes a ucode-based unit test with a fixture file that can be
run locally with: ucode test/nat_traffic.uc

Signed-off-by: Ricard Nacher <redacted>