Allow risk object to be nested under network.

[willem-dhaese]

Please allow risk object to be nested under network.

https://www.elastic.co/guide/en/ecs/current/ecs-risk.html
https://www.elastic.co/guide/en/ecs/current/ecs-network.html

We are in a very segmented network and we need to classify or network zones with a risk score so we can tune our detection rules and focus on network zones with a high risk level.