Adopt container inventory schema fields in kubernetes integration

[kaiyan-sheng]

We are trying to add several new fields into the container inventory schema and ECS: elastic/ecs#1529.
Before getting these fields in officially, let's test them in the kubernetes integration first to see if they are applicable and well defined. After these fields are merged into ECS, we can officially adopt these fields in integrations and Kibana.

[botelastic]

Hi! We just realized that we haven't looked into this issue in a while. We're sorry! We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!